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..bb85e25f31f197608f6eb54483d852250f1a4ab9 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18944,6 +18944,47 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audio.Capture.FramesRequested" units="frames">

[Henrik Grunell, 2016/02/29 17:48:48]

I changed "FramesRequested" to "FramesProvided", since that's what it actually
is.

+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The number of frames audio is actually captured at if the number is

[Mark P, 2016/02/26 17:10:55]

Please proofread.  I can't parse this sentence.  I think you're missing a "that"
or an "as" somewhere or maybe you have an unnecessary "at".  I can't figure out
which version of the sentence you intended.  Also, "frames audio" sounds
awkward.  Try "audio frames" or "frames of audio" or something.

[Henrik Grunell, 2016/02/29 17:48:48]

Done.

+    different from the desired number of frames.  If the desired frame size is

[Mark P, 2016/02/26 17:10:56]

Please briefly explain what "desired frame size" is / when it is used.
That probably will also help me interpret when the statement "this value will be
0" is a natural consequence of using setting a desired frame size or a foible of
your logging.  It may also help me understand whether it makes sense for you to
emit to this histogram at all in this case.

[Henrik Grunell, 2016/02/29 17:48:48]

Done.

+    used, this value will be 0. Typically, Chrome will want to use a 10ms buffer
+    (e.g. 480 frames for a sample rate of 48kHz), but we might have to capture
+    audio 256 frames at a time. This histogram gives us an indication as to how
+    frequent that is and what buffer sizes are used in practice. This value is
+    logged when an audio input stream is stopped.

[Mark P, 2016/02/26 17:10:56]

Are pauses included or does this refer to (basically) closing the stream
entirely?
ditto below.

[Henrik Grunell, 2016/02/29 17:48:48]

Closed. Changed.

+  </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

[Mark P, 2016/02/26 17:10:55]

Consider whether "at the OS level" in this sentence and "if the OS has" (replace
with "we had") adds anything.  It's not clear to me it does.
(Does does it matter which level of the hierarchy detects a glitch?  And if the
OS is the only one that does, than why bother saying it happens at that level?)

ditto below

[Henrik Grunell, 2016/02/29 17:48:48]

It matters - this is OS glitches only.

+    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 stopped.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.Capture.LargestGlitchMs" units="ms">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The length in milliseconds of the largest glitch that was detected at the OS

[Mark P, 2016/02/26 17:10:56]

optional nit: largest glitch -> largest audio glitch

[Henrik Grunell, 2016/02/29 17:48:48]

Done.

+    level. This value is logged when an audio input stream that has suffered
+    glitches is stopped.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.Capture.LostFramesInMs" units="ms">
+  <owner>grunell@chromium.org</owner>
+  <summary>
+    The combined length in milliseconds of audio glitches. 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
+    stopped.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.InputBufferSizeWasChangedMac"
     enum="BooleanSuccess">
   <owner>henrika@chromium.org</owner>