WebRTC: Fix and enable -Woverloaded-virtual warnings.

Essentially applying the same change as in
https://codereview.webrtc.org/2023413002 in more locations.

There's only one change affecting production code: enabling the warning
for webrtc/media:rtc_media. The rest are test changes.

With these changes, the only place the warning is disabled is in the Windows
implementation of webrtc/modules/video_capture:video_capture_internal_impl,
which is harder to fix, since it relies on sample code from the Windows SDK.

BUG=webrtc:6653
NOTRY=True

Review-Url: https://codereview.webrtc.org/2468093004
Cr-Commit-Position: refs/heads/master@{#14938}
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 3f84c3d..be4dc06 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -435,6 +435,13 @@
 
 class MockPeerConnectionObserver : public PeerConnectionObserver {
  public:
+  // We need these using declarations because there are two versions of each of
+  // the below methods and we only override one of them.
+  // TODO(deadbeef): Remove once there's only one version of the methods.
+  using PeerConnectionObserver::OnAddStream;
+  using PeerConnectionObserver::OnRemoveStream;
+  using PeerConnectionObserver::OnDataChannel;
+
   MockPeerConnectionObserver() : remote_streams_(StreamCollection::Create()) {}
   virtual ~MockPeerConnectionObserver() {
   }