Adds Media.Audio.InputDevicePropertyChangedMac UMA stat 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 31c72c0cd68c05263e5cb82969c0f70aa4e3450c..7094df8a1ff7261a580c50d48bc3adb37d9fc65f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18924,6 +18924,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audio.InputDevicePropertyChangedMac"
+    enum="AudioDevicePropertyResult">
+  <owner>henrika@chromium.org</owner>
+  <summary>
+    Lists device properties that have been changed during an audio input stream
+    session. Sampled when an AUAudioInputStream object is closed.

[Ilya Sherman, 2016/02/22 18:38:30]

Please make extra clear that multiple enum values can be emitted each time an
AUAudioInputStream object is closed.  "Sampled" might suggest that a random
property, or perhaps the most relevant property, is chosen among all that apply.

[henrika (OOO until Aug 14), 2016/02/22 18:51:58]

Will change and make that more clear.

[henrika (OOO until Aug 14), 2016/02/23 11:34:53]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Media.Audio.InputDevicePropertyChangedStartupFailedMac"
+    enum="AudioDevicePropertyResult">
+  <owner>henrika@chromium.org</owner>
+  <summary>
+    Lists device properties that have been changed during an audio input stream
+    session. Only sampled when Media.Audio.InputStartupSuccessMac reports
+    'Failure'.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.InputStartupSuccessMac" enum="BooleanSuccess">
   <owner>henrika@chromium.org</owner>
   <summary>
@@ -58215,6 +58234,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="16" label="kCodecAC3"/>
 </enum>
 
+<enum name="AudioDevicePropertyResult" type="int">
+  <int value="0" label="Device has changed"/>
+  <int value="1" label="IO stopped abnormally"/>
+  <int value="2" label="Hog mode"/>
+  <int value="3" label="Bufer frame size"/>
+  <int value="4" label="Buffer frame size range"/>
+  <int value="5" label="Stream configuration"/>
+  <int value="6" label="Actual sample rate"/>
+  <int value="7" label="Nominal sample rate"/>
+  <int value="8" label="Device is running somewhere"/>
+  <int value="9" label="Device is running"/>
+  <int value="10" label="Device is alive"/>
+  <int value="11" label="Stream physical format"/>
+  <int value="12" label="Not specified"/>

[Ilya Sherman, 2016/02/22 18:38:30]

I'd recommend listing this case first, so that it's not buried in the middle if
you add more cases later.  (It's the default case, right?)

[henrika (OOO until Aug 14), 2016/02/22 18:51:58]

It covers a set of property changes that can happen but that we don't log since
they are not related to the problem we are tracking. I see this as the "default"
in a switch/case statement hence it felt natural to use as last value.

[Ilya Sherman, 2016/02/22 19:30:16]

Sorry, let me be clearer: Currently, you've assigned the label "other" to the
backing value of "12".  That makes sense while you have 12 known device
properties.  What happens if you add a 13th known device property?  You can't
renumber "other" to now have value 13, because the semantics of enumerated
values used to back histograms are expected to remain consistent over time.  So,
you'd have to add this property after the "other" property... which would leave
"other" strangely buried in the middle.  That's why I'm suggesting moving it up
to the front to begin with: It's a form of future-proofing =)

[henrika (OOO until Aug 14), 2016/02/23 11:34:53]

Thanks for the detailed description. See your point now ;-)

+</enum>
+
 <enum name="AudioFramesPerBuffer" type="int">
   <int value="0" label="k160"/>
   <int value="1" label="k320"/>