Add UMA for droppedframes count

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 a044f20556b4f94ee35c4535ccf96f5174affb68..4a0f042fe521b9694cd0e80f67d952fbbc6830c0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10962,6 +10962,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Media.DroppedFrameCount">
+  <owner>prabhur@chromium.org</owner>
+  <summary>Count of dropped frames between pipeline start and stop.</summary>

[Ilya Sherman, 2014/08/22 23:53:08]

For those of us not familiar with the media code, it might be nice to give some
idea of what causes a pipeline to start and stop.

[prabhur1, 2014/08/23 00:23:06]

Done.

+</histogram>

[Ilya Sherman, 2014/08/22 23:53:08]

It's often a good idea to compare a failure rate to a success rate, so that you
get a relative failure rate.  Absolute numbers tend to be harder to interpret.

OTOH, if you think that dropped frames ought to be quite rare, then maybe this
histogram on its own could give you a good indication as to whether your
hypothesis is correct.

[prabhur1, 2014/08/23 00:23:06]

Dropped frame count is expected to be close to 0 in most cases. Any huge spikes
need to be investigated. There are exceptions but in general it is expected to
be around 0.

[prabhur1, 2014/08/23 00:23:06]

Acknowledged.

+
 <histogram name="Media.Duration" units="ms">
   <owner>scherkus@chromium.org</owner>
   <summary>Duration in milliseconds of HTML5 media (when known).</summary>