Add UMA for droppedframes count

Adding droppedframecount UMA to monitor videoplayback quality. Metric gets logged only if the pipeline has video & has decoded atleast one video frame.

BUG=
R=dalecurtis

Review URL: https://codereview.chromium.org/497913002

Cr-Commit-Position: refs/heads/master@{#292449}

media/base/pipeline.cc

@@ -530,6 +530,10 @@ void Pipeline::StopTask(const base::Closure& stop_cb) {
   if (state_ == kStopping)
     return;
 
+  PipelineStatistics stats = GetStatistics();
+  if (renderer_ && renderer_->HasVideo() && stats.video_frames_decoded > 0)
+    UMA_HISTOGRAM_COUNTS("Media.DroppedFrameCount", stats.video_frames_dropped);
+
   SetState(kStopping);
   pending_callbacks_.reset();
   DoStop(base::Bind(&Pipeline::OnStopCompleted, weak_factory_.GetWeakPtr()));

tools/metrics/histograms/histograms.xml

@@ -11085,6 +11085,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Media.DroppedFrameCount">
+  <owner>prabhur@chromium.org</owner>
+  <summary>
+    Count of dropped frames between pipeline start and stop. Media pipeline
+    starts/stops when an HTML5 video is loaded/unloaded respectively in the
+    browser.
+  </summary>
+</histogram>
+
 <histogram name="Media.Duration" units="ms">
   <owner>scherkus@chromium.org</owner>
   <summary>Duration in milliseconds of HTML5 media (when known).</summary>