Add FramesSentAsOverlay UMA metric to AVDA.

This metric records frames sent by AVDA, bucketed by whether they
are or are not overlays.

The goal is to provide an approximate count of how often SurfaceView
is used versus SurfaceTexture.

BUG=629313

Review-Url: https://codereview.chromium.org/2163513002
Cr-Commit-Position: refs/heads/master@{#406712}

media/gpu/android_video_decode_accelerator.cc

@@ -939,6 +939,7 @@ void AndroidVideoDecodeAccelerator::SendDecodedFrameToClient(
   }
 
   const bool allow_overlay = strategy_->ArePicturesOverlayable();
+  UMA_HISTOGRAM_BOOLEAN("Media.AVDA.FrameSentAsOverlay", allow_overlay);
   Picture picture(picture_buffer_id, bitstream_id, gfx::Rect(size_),
                   allow_overlay);
   picture.set_size_changed(size_changed);

tools/metrics/histograms/histograms.xml

@@ -21682,6 +21682,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.AVDA.FrameSentAsOverlay" enum="BooleanFrameAsOverlay">
+  <owner>liberato@chromium.org</owner>
+  <summary>
+    Record a count for each frame sent to the client by AVDA.  True counts
+    indicate that the frame was an overlay (SurfaceView).  False counts are for
+    SurfaceTexture frames.
+  </summary>
+</histogram>
+
 <histogram name="Media.AVDA.InputQueueTime" units="ms">
   <owner>watk@chromium.org</owner>
   <obsolete>
@@ -67712,6 +67721,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Lacks Form Manager"/>
 </enum>
 
+<enum name="BooleanFrameAsOverlay" type="int">
+  <int value="0" label="Frame was not allow_overlay"/>
+  <int value="1" label="Frame was allow_overlay"/>
+</enum>
+
 <enum name="BooleanFromAddressBook" type="int">
   <int value="0" label="Not From Address Book"/>
   <int value="1" label="From Address Book"/>