Android: Replace webrtc_jni namespace with nested jni namespace

This CL replaces:
namespace webrtc_jni {
with:
namespace webrtc {
namespace jni {

The main benefit is that we don't have to use the webrtc:: qualifier
inside the jni namespace so we can reduce some clutter.

BUG=None

Review-Url: https://codereview.webrtc.org/3009613002
Cr-Commit-Position: refs/heads/master@{#19569}
diff --git a/webrtc/examples/unityplugin/classreferenceholder.cc b/webrtc/examples/unityplugin/classreferenceholder.cc
index 8814699..b009ce4 100644
--- a/webrtc/examples/unityplugin/classreferenceholder.cc
+++ b/webrtc/examples/unityplugin/classreferenceholder.cc
@@ -36,12 +36,12 @@
 
 void LoadGlobalClassReferenceHolder() {
   RTC_CHECK(g_class_reference_holder == nullptr);
-  g_class_reference_holder = new ClassReferenceHolder(webrtc_jni::GetEnv());
+  g_class_reference_holder = new ClassReferenceHolder(webrtc::jni::GetEnv());
 }
 
 void FreeGlobalClassReferenceHolder() {
   g_class_reference_holder->FreeReferences(
-      webrtc_jni::AttachCurrentThreadIfNeeded());
+      webrtc::jni::AttachCurrentThreadIfNeeded());
   delete g_class_reference_holder;
   g_class_reference_holder = nullptr;
 }
diff --git a/webrtc/examples/unityplugin/classreferenceholder.h b/webrtc/examples/unityplugin/classreferenceholder.h
index a3f83e8..7d9a71b 100644
--- a/webrtc/examples/unityplugin/classreferenceholder.h
+++ b/webrtc/examples/unityplugin/classreferenceholder.h
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-// This is a supplement of webrtc_jni::ClassReferenceHolder.
+// This is a supplement of webrtc::jni::ClassReferenceHolder.
 // The purpose of this ClassReferenceHolder is to load the example
 // specific java class into JNI c++ side, so that our c++ code can
 // call those java functions.
diff --git a/webrtc/examples/unityplugin/jni_onload.cc b/webrtc/examples/unityplugin/jni_onload.cc
index b8b4adf..558dcfb 100644
--- a/webrtc/examples/unityplugin/jni_onload.cc
+++ b/webrtc/examples/unityplugin/jni_onload.cc
@@ -17,7 +17,8 @@
 #include "webrtc/sdk/android/src/jni/classreferenceholder.h"
 #include "webrtc/sdk/android/src/jni/jni_helpers.h"
 
-namespace webrtc_jni {
+namespace webrtc {
+namespace jni {
 
 extern "C" jint JNIEXPORT JNICALL JNI_OnLoad(JavaVM* jvm, void* reserved) {
   jint ret = InitGlobalJniVariables(jvm);
@@ -38,4 +39,5 @@
   RTC_CHECK(rtc::CleanupSSL()) << "Failed to CleanupSSL()";
 }
 
-}  // namespace webrtc_jni
+}  // namespace jni
+}  // namespace webrtc
diff --git a/webrtc/examples/unityplugin/simple_peer_connection.cc b/webrtc/examples/unityplugin/simple_peer_connection.cc
index 57ab885..3f3f3fb 100644
--- a/webrtc/examples/unityplugin/simple_peer_connection.cc
+++ b/webrtc/examples/unityplugin/simple_peer_connection.cc
@@ -164,10 +164,10 @@
 
 #if defined(WEBRTC_ANDROID)
   if (g_camera) {
-    JNIEnv* env = webrtc_jni::GetEnv();
+    JNIEnv* env = webrtc::jni::GetEnv();
     jclass pc_factory_class =
         unity_plugin::FindClass(env, "org/webrtc/UnityUtility");
-    jmethodID stop_camera_method = webrtc_jni::GetStaticMethodID(
+    jmethodID stop_camera_method = webrtc::jni::GetStaticMethodID(
         env, pc_factory_class, "StopCamera", "(Lorg/webrtc/VideoCapturer;)V");
 
     env->CallStaticVoidMethod(pc_factory_class, stop_camera_method, g_camera);
@@ -409,10 +409,10 @@
 
   if (!audio_only) {
 #if defined(WEBRTC_ANDROID)
-    JNIEnv* env = webrtc_jni::GetEnv();
+    JNIEnv* env = webrtc::jni::GetEnv();
     jclass pc_factory_class =
         unity_plugin::FindClass(env, "org/webrtc/UnityUtility");
-    jmethodID load_texture_helper_method = webrtc_jni::GetStaticMethodID(
+    jmethodID load_texture_helper_method = webrtc::jni::GetStaticMethodID(
         env, pc_factory_class, "LoadSurfaceTextureHelper",
         "()Lorg/webrtc/SurfaceTextureHelper;");
     jobject texture_helper = env->CallStaticObjectMethod(
@@ -421,15 +421,15 @@
     RTC_DCHECK(texture_helper != nullptr)
         << "Cannot get the Surface Texture Helper.";
 
-    rtc::scoped_refptr<webrtc::AndroidVideoTrackSource> source(
-        new rtc::RefCountedObject<webrtc::AndroidVideoTrackSource>(
+    rtc::scoped_refptr<AndroidVideoTrackSource> source(
+        new rtc::RefCountedObject<AndroidVideoTrackSource>(
             g_signaling_thread.get(), env, texture_helper, false));
     rtc::scoped_refptr<webrtc::VideoTrackSourceProxy> proxy_source =
         webrtc::VideoTrackSourceProxy::Create(g_signaling_thread.get(),
                                               g_worker_thread.get(), source);
 
     // link with VideoCapturer (Camera);
-    jmethodID link_camera_method = webrtc_jni::GetStaticMethodID(
+    jmethodID link_camera_method = webrtc::jni::GetStaticMethodID(
         env, pc_factory_class, "LinkCamera",
         "(JLorg/webrtc/SurfaceTextureHelper;)Lorg/webrtc/VideoCapturer;");
     jobject camera_tmp =