Enable GN check for webrtc/{p2p,system_wrappers}

Introduce new small header-only targets in system_wrappers:
:cpu_features_api
:field_trial_api
:metrics_api
to untangle and optimize dependencies but still satisfy GN check.

In webrtc/p2p, previously uncovered header "base/fakecandidatepair.h"
is added to :p2p_test_utils target.

Refactor system_wrappers so 'rtc_p2p' can depend on only
system_wrappers:field_trial_api instead of all of system_wrappers
(which led to a breakage in Chromium that called for the revert of
https://codereview.webrtc.org/2735583002).

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2739863002
Cr-Commit-Position: refs/heads/master@{#17812}
diff --git a/webrtc/p2p/BUILD.gn b/webrtc/p2p/BUILD.gn
index a31b7f1..3141c52 100644
--- a/webrtc/p2p/BUILD.gn
+++ b/webrtc/p2p/BUILD.gn
@@ -85,7 +85,10 @@
   defines = []
 
   deps = [
+    "../api:libjingle_peerconnection_api",
+    "../api:ortc_api",
     "../base:rtc_base",
+    "../system_wrappers:field_trial_api",
   ]
 
   if (rtc_build_expat) {
@@ -145,6 +148,7 @@
   rtc_source_set("p2p_test_utils") {
     testonly = true
     sources = [
+      "base/fakecandidatepair.h",
       "base/fakedtlstransport.h",
       "base/fakeicetransport.h",
       "base/fakepackettransport.h",
@@ -155,16 +159,15 @@
       "base/teststunserver.h",
       "base/testturnserver.h",
     ]
-
     deps = [
       ":rtc_p2p",
+      "../api:ortc_api",
+      "../base:rtc_base",
+      "../base:rtc_base_approved",
+      "../base:rtc_base_tests_utils",
+      "../test:test_support",
       "//testing/gmock",
     ]
-
-    if (!build_with_chromium && is_clang) {
-      # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
-      suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
-    }
   }
 
   rtc_source_set("rtc_p2p_unittests") {
@@ -205,6 +208,11 @@
       ":p2p_test_utils",
       ":rtc_p2p",
       "../api:fakemetricsobserver",
+      "../api:ortc_api",
+      "../base:rtc_base",
+      "../base:rtc_base_approved",
+      "../base:rtc_base_tests_utils",
+      "../test:test_support",
       "//testing/gmock",
       "//testing/gtest",
     ]
@@ -228,6 +236,7 @@
   }
 
   deps = [
+    ":rtc_p2p",
     "..:webrtc_common",
     "../base:rtc_base",
   ]
@@ -241,6 +250,10 @@
     ]
     deps = [
       ":libstunprober",
+      ":p2p_test_utils",
+      ":rtc_p2p",
+      "../base:rtc_base",
+      "../base:rtc_base_tests_utils",
       "//testing/gmock",
       "//testing/gtest",
     ]