media: Add UMA stats for bytes going in and out of multibuffer cache

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 0457c56e5fa066cf550e55422ad77f3e072fc0e5..269726506fcf854110eee967f4ea4a77d5b2c73c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29246,6 +29246,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.BytesReadFromCache" units="KB">
+  <owner>hubbe@chromium.org</owner>
+  <owner>media-dev@chromium.org</owner>
+  <summary>Kb read by media demuxer from MultiBuffer cache.</summary>

[rkaplow1, 2017/07/18 18:54:05]

may want to make it more clear when this is recorded.

[hubbe, 2017/07/19 18:00:58]

It's not an easy thing to explain succinctly.
(As is often the case when you have a cache with lazy eviction..)

+</histogram>
+
+<histogram name="Media.BytesReadFromNetwork" units="KB">
+  <owner>hubbe@chromium.org</owner>
+  <owner>media-dev@chromium.org</owner>
+  <summary>Kb read by from network into MultiBuffer cache.</summary>
+</histogram>
+
 <histogram name="Media.CacheUseful" enum="BooleanSuccess">
   <owner>scherkus@chromium.org</owner>
   <summary>