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/media/BUILD.gn b/webrtc/media/BUILD.gn
index 9689cd2..a00fe6f 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -229,6 +229,7 @@
       "base/fakertp.h",
       "base/fakevideocapturer.h",
       "base/fakevideorenderer.h",
+      "base/test/mock_mediachannel.h",
       "base/testutils.cc",
       "base/testutils.h",
       "engine/fakewebrtccall.cc",