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}
