Add UMA stats for silence AudioInputController.

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 7fe917f2fc57278cf9fee089ffb6c460b7b35fbf..f4d574c495452d296f49bbb5e97d12822688924b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10737,6 +10737,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Media.AudioInputControllerSessionSilenceReport"
+    enum="AudioInputSilenceReport">
+  <owner>grunell@chromium.org</owner>
+  <summary>The silence report for an audio input device session.</summary>
+</histogram>
+
 <histogram name="Media.AudioInputDeviceManager" units="ms">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>Measures the time taken for AudioInputDeviceManager::</summary>
@@ -37064,6 +37070,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="8" label="k1920"/>
 </enum>
 
+<enum name="AudioInputSilenceReport" type="int">
+  <int value="0" label="No measurement"/>
+  <int value="1" label="Only audio"/>
+  <int value="2" label="Only silence"/>
+  <int value="3" label="Audio and silence"/>
+</enum>
+
 <enum name="AudioRendererEvents" type="int">
   <int value="0" label="Initialized"/>
   <int value="1" label="Runtime error"/>