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..563cb10893f601c5755f7e3eaf599ca5556da9ee 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.EstimatedDroppableEncodedSize" units="KB">
+  <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.  This is
+    rencorded when an ImageResource is pruned.  See crbug/664437.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Renderer.Large2" units="MB">
   <owner>bashi@chromium.org</owner>
   <owner>kouhei@chromium.org</owner>