WebRTC: Fix and enable -Woverloaded-virtual warnings.

webrtc/api/BUILD.gn

 # Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../build/webrtc.gni")
 if (is_android) {
@@ skipping 342 matching lines @@
       cflags = [
         # See https://bugs.chromium.org/p/webrtc/issues/detail?id=6267
         # for -Wno-sign-compare
         "-Wno-sign-compare",
         "-Wno-unused-function",
       ]
     }
 
     if (!is_win) {
       cflags = [ "-Wno-sign-compare" ]
-      cflags_cc = [ "-Wno-overloaded-virtual" ]
     }
   }
 
   rtc_test("peerconnection_unittests") {
     testonly = true
     sources = [
       "datachannel_unittest.cc",
       "dtmfsender_unittest.cc",
       "fakemetricsobserver.cc",
       "fakemetricsobserver.h",
@@ skipping 117 matching lines @@
 
       shared_libraries = [ ":libjingle_peerconnection_so" ]
     }
 
     android_resources("libjingle_peerconnection_android_unittest_resources") {
       resource_dirs = [ "androidtests/res" ]
       custom_package = "org.webrtc"
     }
   }
 }