Add histogram WebRTC.UserMediaRequest.Events to allow to track getUserMedia failures and hangs.

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 0ef82fff9f2756cf671835d0477609ee582d3078..c9286466a6095edea4cb11aa06a0bb8482a2e347 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35607,6 +35607,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.UserMediaRequest.Result"
+    enum="MediaStreamRequestResult">
+  <owner>andresp@chromium.org</owner>
+  <summary>
+    Counters for UserMediaRequests results such as failure reasons.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.webkitApiCount" enum="JavaScriptAPIName">
   <owner>perkj@chromium.org</owner>
   <summary>Counts number of calls to WebRTC APIs from JavaScript.</summary>
@@ -43055,6 +43063,18 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="2" label="All external links protected"/>
 </enum>
 
+<enum name="MediaStreamRequestResult" type="int">
+  <int value="0" label="Ok"/>
+  <int value="1" label="Permission Denied"/>
+  <int value="2" label="Permission Dismissed"/>
+  <int value="3" label="Invalid State"/>
+  <int value="4" label="No Hardware"/>
+  <int value="5" label="Invalid Security Origin"/>
+  <int value="6" label="Tab Capture Failure"/>
+  <int value="7" label="Capture Failure"/>
+  <int value="8" label="Track Start Failure"/>
+</enum>
+
 <enum name="MetaTagTypeEnum" type="int">
   <int value="0" label="No viewport tag"/>
   <int value="1" label="Viewport meta with device width"/>