Cleanup: Move json.h into rtc namespace.

This should fix the TODO in that header.

BUG=None
TEST=ninja -C out/Debug still compiles everything.
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47919004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#8921}
diff --git a/webrtc/base/json.cc b/webrtc/base/json.cc
index 49a051c..c692534 100644
--- a/webrtc/base/json.cc
+++ b/webrtc/base/json.cc
@@ -16,6 +16,8 @@
 
 #include <sstream>
 
+namespace rtc {
+
 bool GetStringFromJson(const Json::Value& in, std::string* out) {
   if (!in.isString()) {
     std::ostringstream s;
@@ -294,3 +296,5 @@
   std::string value = w.write(json);
   return value.substr(0, value.size() - 1);  // trim trailing newline
 }
+
+}  // namespace rtc