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

content/renderer/media/webrtc_uma_histograms.cc

Index: content/renderer/media/webrtc_uma_histograms.cc
diff --git a/content/renderer/media/webrtc_uma_histograms.cc b/content/renderer/media/webrtc_uma_histograms.cc
index 16da393073e45e1b27d8f4e055faa0a898d7f9f2..6dc50a397c8e7f222128d2393f3cc73f7a900254 100644
--- a/content/renderer/media/webrtc_uma_histograms.cc
+++ b/content/renderer/media/webrtc_uma_histograms.cc
@@ -8,6 +8,11 @@
 
 namespace content {
 
+void LogUserMediaRequestResult(MediaStreamRequestResult result) {
+  UMA_HISTOGRAM_ENUMERATION(
+      "WebRTC.UserMediaRequest.Result", result, NUM_MEDIA_REQUEST_RESULTS);
+}
+
 void UpdateWebRTCMethodCount(JavaScriptAPIName api_name) {
   DVLOG(3) << "Incrementing WebRTC.webkitApiCount for " << api_name;
   UMA_HISTOGRAM_ENUMERATION("WebRTC.webkitApiCount", api_name, INVALID_NAME);