Add histogram WebRTC.UserMediaRequest.NoResultState to track user media requests with no result.

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 c9286466a6095edea4cb11aa06a0bb8482a2e347..912f7abdcc4d6a7f9cc65288c39155f51f0a384d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35607,6 +35607,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.UserMediaRequest.NoResultState"
+    enum="MediaStreamRequestState">
+  <owner>andresp@chromium.org</owner>
+  <summary>
+    Counters for state of UserMediaRequests when they get destroyed before

[Mark P, 2014/08/05 18:50:23]

Consider:
Counters for state of UserMediaRequests when they get
...
->
The state of a UserMediaRequest when it gets
...

[andresp-chromium, 2014/08/06 10:13:56]

Done.

+    having a result.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.UserMediaRequest.Result"
     enum="MediaStreamRequestResult">
   <owner>andresp@chromium.org</owner>
@@ -43075,6 +43084,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="8" label="Track Start Failure"/>
 </enum>
 
+<enum name="MediaStreamRequestState" type="int">
+  <int value="0" label="Cancelled"/>
+  <int value="1" label="Stream Not Generated"/>
+  <int value="2" label="Pending Media Tracks"/>
+</enum>
+
 <enum name="MetaTagTypeEnum" type="int">
   <int value="0" label="No viewport tag"/>
   <int value="1" label="Viewport meta with device width"/>