Add WebRTC histograms: dropped video frames on receiver

The metrics themselves are added in the separate CL in webrtc.

Bug: webrtc:8355
Change-Id: Ic7d98d7e1f5b30d62a79418b110e3e9c7123008f
Reviewed-on: https://chromium-review.googlesource.com/921221Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537276}

tools/metrics/histograms/histograms.xml

@@ -100583,6 +100583,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Video.DroppedFrames.Receiver" units="frames">
+  <owner>ilnik@chromium.org</owner>
+  <summary>
+    Total number of frames dropped by a WebRTC on the receive side because they
+    are incomplete or undecodable.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Video.DuplicatedPacketsInPercent" units="%">
   <owner>asapersson@chromium.org</owner>
   <summary>
@@ -101068,6 +101076,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Video.Screenshare.DroppedFrames.Receiver"
+    units="frames">
+  <owner>ilnik@chromium.org</owner>
+  <summary>
+    Total number of frames dropped by a WebRTC on the receive side because they
+    are incomplete or undecodable.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Video.Screenshare.EncodeTimeInMs" units="ms">
   <owner>sprang@chromium.org</owner>
   <summary>
@@ -113549,6 +113566,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <suffix name="S1"/>
   <suffix name="S2"/>
   <affected-histogram name="WebRTC.Video.Decoded.Vp8.Qp"/>
+  <affected-histogram name="WebRTC.Video.DroppedFrames.Receiver"/>
   <affected-histogram name="WebRTC.Video.EndToEndDelayInMs"/>
   <affected-histogram name="WebRTC.Video.EndToEndDelayMaxInMs"/>
   <affected-histogram name="WebRTC.Video.InterframeDelay95PercentileInMs"/>
@@ -113559,6 +113577,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="WebRTC.Video.ReceivedHeightInPixels"/>
   <affected-histogram name="WebRTC.Video.ReceivedWidthInPixels"/>
   <affected-histogram name="WebRTC.Video.Screenshare.Decoded.Vp8.Qp"/>
+  <affected-histogram name="WebRTC.Video.Screenshare.DroppedFrames.Receiver"/>
   <affected-histogram name="WebRTC.Video.Screenshare.EndToEndDelayInMs"/>
   <affected-histogram name="WebRTC.Video.Screenshare.EndToEndDelayMaxInMs"/>
   <affected-histogram