Add UMA to estimate deroppable memory usage of encoded data size in Resources

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 fab6407492b9bfd85448a80bd3a48bb22b8d3541..e02927ebc36b635997e4fe71587d52141d3f67b8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25919,6 +25919,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Renderer.EstimantedDroppableEncodedSize" units="KB">

[Ilya Sherman, 2016/11/16 06:33:35]

nit: s/Estimanted/Estimated (please double-check the name in the C++ code as
well)

[hajimehoshi, 2016/11/16 09:59:10]

Done.

+  <owner>hajimehoshi@chromium.org</owner>
+  <summary>
+    The esitimated memory size which would be reduced when reusing disk cache is
+    implemented.  If it is 100% sure that the encoded data is in disk cache, the
+    same data in memory can be dropped to reduce memory usage.  See
+    crbug/664437.

[Ilya Sherman, 2016/11/16 06:33:35]

Please document when this metrics is recorded.

[hajimehoshi, 2016/11/16 09:59:10]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Memory.Renderer.Large2" units="MB">
   <owner>bashi@chromium.org</owner>
   <owner>kouhei@chromium.org</owner>