content: Add histogram for discardable memory allocation size.

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 4fd69481473431e2954fa9b5ccbb37c6646fafb8..98099515c3d5b36245282b7d5682f4ccf8534bf8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14066,6 +14066,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Memory.DiscardableAllocationSize" units="KB">

[Alexei Svitkine (slow), 2015/03/09 14:15:44]

Nit: Maybe name it Memory.DiscardableAllocationSizeKB

[reveman, 2015/03/09 15:12:40]

There doesn't seem to be any general consistency in regards to including the
unit in the name or not. I would prefer not to include it to be consistent with
other Memory.* histograms and it seems a bit superfluous to have it in the name
when we have units="KB". Or am I missing some good reason for having it in the
name?

[Alexei Svitkine (slow), 2015/03/09 15:17:13]

I think not all our dashboards show the units text. If you prefer to leave out
of the name, I would at least mention it in the description. (i.e. "allocation
size in KB.")

[reveman, 2015/03/09 15:24:16]

Ok, thanks. Done.

+  <owner>reveman@chromium.org</owner>
+  <summary>
+    The discardable memory allocation size. Recorded each time a new discardable
+    memory instance is created.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Extension" units="KB">
   <owner>hajimehoshi@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>