RTCMediaStreamTrackStats.framesDecoded collected.

According to spec:
https://w3c.github.io/webrtc-stats/#dom-rtcmediastreamtrackstats-framesdecoded

BUG=webrtc:6757, chromium:659137, chromium:627816

Review-Url: https://codereview.webrtc.org/2642713004
Cr-Commit-Position: refs/heads/master@{#16192}
diff --git a/webrtc/api/rtcstatscollector_unittest.cc b/webrtc/api/rtcstatscollector_unittest.cc
index da5d41e..c5e8a9c 100644
--- a/webrtc/api/rtcstatscollector_unittest.cc
+++ b/webrtc/api/rtcstatscollector_unittest.cc
@@ -1543,6 +1543,7 @@
   video_receiver_info_ssrc3.frame_width = 6789;
   video_receiver_info_ssrc3.frame_height = 9876;
   video_receiver_info_ssrc3.frames_received = 2468;
+  video_receiver_info_ssrc3.frames_decoded = 1234;
 
   // Remote video track with undefined (default) values
   rtc::scoped_refptr<MediaStreamTrackInterface> remote_video_track_ssrc4 =
@@ -1557,6 +1558,7 @@
   video_receiver_info_ssrc4.frame_width = 0;
   video_receiver_info_ssrc4.frame_height = 0;
   video_receiver_info_ssrc4.frames_received = 0;
+  video_receiver_info_ssrc4.frames_decoded = 0;
 
   test_->CreateMockRtpSendersReceiversAndChannels(
       {}, {},
@@ -1632,6 +1634,7 @@
   expected_remote_video_track_ssrc3.frame_width = 6789;
   expected_remote_video_track_ssrc3.frame_height = 9876;
   expected_remote_video_track_ssrc3.frames_received = 2468;
+  expected_remote_video_track_ssrc3.frames_decoded = 1234;
   ASSERT_TRUE(report->Get(expected_remote_video_track_ssrc3.id()));
   EXPECT_EQ(expected_remote_video_track_ssrc3,
             report->Get(expected_remote_video_track_ssrc3.id())->cast_to<
@@ -1646,6 +1649,7 @@
   expected_remote_video_track_ssrc4.ended = false;
   expected_remote_video_track_ssrc4.detached = false;
   expected_remote_video_track_ssrc4.frames_received = 0;
+  expected_remote_video_track_ssrc4.frames_decoded = 0;
   // Should be undefined: |expected_remote_video_track_ssrc4.frame_width| and
   // |expected_remote_video_track_ssrc4.frame_height|.
   ASSERT_TRUE(report->Get(expected_remote_video_track_ssrc4.id()));