Add a histogram for video decoder fallback.

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 6a2c26fcc6bed77f1899a56205c5cbe18f40233b..bf96a086be8f84c2189950889120c23a1a1266d6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12900,6 +12900,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Media.VideoDecoderFallback" enum="BooleanSuccess">
+  <owner>watk@chromium.org</owner>
+  <summary>
+    Whether Chrome was able to fall back to another video decoder when a
+    previously intialized one failed to reinitialize. Failure is recorded when a

[DaleCurtis, 2015/01/13 19:20:50]

Everything after "Failure..." seems a bit more than necessary.  We generally try
to keep these concise.  How about: "Whether Chrome had to fallback to a
secondary decoder after the primary decoder failed reinitialization."

+    video decoder is successfully initialized, so fallback will result in both a
+    Success and Failure being recorded.
+  </summary>
+</histogram>
+
 <histogram name="Media.VideoPixelFormat" enum="VideoPixelFormat">
   <owner>scherkus@chromium.org</owner>
   <summary>Pixel format used in HTML5 video. Emitted on video load.</summary>