Moving mock classes around so that they may be reused in other unittests

New files, classes moved from statscollector_unittest.cc:
+webrtc/api/test/mock_peerconnection.h
 for MockPeerConnectionFactory and MockPeerConnection
+webrtc/api/test/mock_webrtcsession.h
 for MockWebRtcSession
+webrtc/media/base/test/mock_mediachannel.h
 for MockVideoMediaChannel and MockVoiceMediaChannel

The webrtc/media/base/test folder is new.

BUG=chromium:627816

Review-Url: https://codereview.webrtc.org/2238933002
Cr-Commit-Position: refs/heads/master@{#13769}
diff --git a/webrtc/api/api_tests.gyp b/webrtc/api/api_tests.gyp
index 5ff062a..73aebaa 100644
--- a/webrtc/api/api_tests.gyp
+++ b/webrtc/api/api_tests.gyp
@@ -49,6 +49,8 @@
         'test/fakeperiodicvideocapturer.h',
         'test/fakertccertificategenerator.h',
         'test/fakevideotrackrenderer.h',
+        'test/mock_peerconnection.h',
+        'test/mock_webrtcsession.h',
         'test/mockpeerconnectionobservers.h',
         'test/peerconnectiontestwrapper.h',
         'test/peerconnectiontestwrapper.cc',