UMA stats for browser/renderer audio rendering buffer size mismatch.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1e85229ab2098cb3036c58fc450a568310f3df3b..2f877281805f0d3ea8806265b1580b63072d455a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21868,6 +21868,21 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audio.Render.BufferSizeMismatch">
+  <owner>olka@chromium.org</owner>
+  <summary>
+    Mismatch between the system buffer size the browser side renders audio at
+    and the buffer size it requests from renderer.  When equals zero, it means
+    buffer sizes are equal. When positive, the value reflects the average number
+    of extra consecutive render requests issued by browser to renderer (scaled
+    by a factor of 1000);  any positive values can cause problems for RTC. When
+    negative, it reflects the average number of extra system audio render calls
+    per one request to the renderer (scaled by a factor of 1000). Any values
+    those are not multiples of 1000 mean that from time to time browser skips a
+    call to renderer, which can cause problems for RTC.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.Render.FramesRequested" units="frames">
   <owner>tommi@chromium.org</owner>
   <summary>