Add UMA stats for OS input glitches on Mac.

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 ca6b8c62374959355279357142603ea2a58786d2..73c30dc01a0a2e216c57063dfc0ef2e777547110 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18944,6 +18944,51 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audio.Capture.FramesProvided" units="frames">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The number of captured audio frames that is provided by the operating system
+    in each callback. Typically, Chrome will want to use a 10 ms buffer (e.g.
+    480 frames for a sample rate of 48 kHz), but the OS may decide to capture
+    e.g. 256 frames of audio at a time. If the desired frame size is used, this
+    value will be 0. This histogram then shows how frequent it is that the

[Mark P, 2016/03/01 19:27:53]

nit for clarity: this value will -> the emitted value is special-cased to

[Henrik Grunell, 2016/03/01 19:44:51]

Done.

+    desired number of frames is used and if not, the number of frames used in
+    practice. The number of frames per callback is normally constant or very
+    rarely changed throughout the lifetime of the audio stream. The first value
+    that differs from the desired value is stored and logged when an audio input
+    stream is closed. Any further differences during the lifetime will be
+    ignored.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.Capture.Glitches" units="glitches">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The number of glitches that were detected at the OS level while an audio
+    stream was active. A glitch is detected if the OS has to skip providing a
+    set of frames. This value is logged when an audio input stream is closed.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.Capture.LargestGlitchMs" units="ms">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The length in milliseconds of the largest audio glitch that was detected at
+    the OS level. This value is logged when an audio input stream that has
+    suffered glitches is closed.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.Capture.LostFramesInMs" units="ms">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The combined length in milliseconds of audio glitches that was detected at
+    the OS level. This is useful to know in particular for audio processing such
+    as echo cancellation. This value is logged when an audio input stream that
+    has suffered glitches is closed.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.InputBufferSizeWasChangedMac"
     enum="BooleanSuccess">
   <owner>henrika@chromium.org</owner>