Add UMA stats for OS input glitches on Mac.

media/audio/mac/audio_auhal_mac.cc

Index: media/audio/mac/audio_auhal_mac.cc
diff --git a/media/audio/mac/audio_auhal_mac.cc b/media/audio/mac/audio_auhal_mac.cc
index acd17bd44ef354d1800fef8ef086c8fff360f77a..d71673fd12ae517dd61f0fdedabda64d2e7e4077 100644
--- a/media/audio/mac/audio_auhal_mac.cc
+++ b/media/audio/mac/audio_auhal_mac.cc
@@ -213,6 +213,9 @@ OSStatus AUHALStream::Render(
   if (number_of_frames != number_of_frames_) {
     // Create a FIFO on the fly to handle any discrepancies in callback rates.
     if (!audio_fifo_) {
+      // TODO(grunell): We'll only care about the first buffer size change,
+      // any further changes will be ignored. It would be nice to have all
+      // changes reflected in UMA stats.
       number_of_frames_requested_ = number_of_frames;
       DVLOG(1) << "Audio frame size changed from " << number_of_frames_
                << " to " << number_of_frames