Add fine grained dropped video frames counters on sending side

4 new counters added to SendStatisticsProxy and reported to UMA and logs.

Bug: webrtc:8355
Change-Id: Idf9b8dfc295c92821e058a97cb3894dc6a446082
Reviewed-on: https://webrtc-review.googlesource.com/12260
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20347}

modules/video_coding/include/video_coding_defines.h

@@ -45,6 +45,8 @@ enum {
   // |kDefaultOutliserFrameSizePercent| in size of average frame.
   kDefaultTimingFramesDelayMs = 200,
   kDefaultOutlierFrameSizePercent = 250,
+  // Maximum number of frames for what we store encode start timing information.
+  kMaxEncodeStartTimeListSize = 50,
 };
 
 enum VCMVideoProtection {