Report timing frames info in GetStats.

Some frames are already marked as 'timing frames' via video-timing RTP header extension. Timestamps along full WebRTC pipeline are gathered for these frames. This CL implements reporting of these timestamps for a single
timing frame since the last GetStats(). The frame with the longest end-to-end delay between two consecutive GetStats calls is reported.

The purpose of this timing information is not to provide a realtime statistics but to provide debugging information as it will help identify problematic places in video pipeline for outliers (frames which took longest to process).

BUG=webrtc:7594

Review-Url: https://codereview.webrtc.org/2946413002
Cr-Commit-Position: refs/heads/master@{#18909}

webrtc/modules/video_coding/frame_buffer2_unittest.cc

@@ -116,6 +116,7 @@ class VCMReceiveStatisticsCallbackMock : public VCMReceiveStatisticsCallback {
                     int jitter_buffer_ms,
                     int min_playout_delay_ms,
                     int render_delay_ms));
+  MOCK_METHOD1(OnTimingFrameInfoUpdated, void(const TimingFrameInfo& info));
 };
 
 class TestFrameBuffer2 : public ::testing::Test {