Add histograms to ResourceFetcher to measure memory cache hit rate.

Source/core/fetch/Resource.h

Index: Source/core/fetch/Resource.h
diff --git a/Source/core/fetch/Resource.h b/Source/core/fetch/Resource.h
index 6d5526b8db38b98471d56c4fdcf1f900af7ce760..0492d62d8d83162d9a1a3c28ae366a944e08ac15 100644
--- a/Source/core/fetch/Resource.h
+++ b/Source/core/fetch/Resource.h
@@ -70,7 +70,8 @@ public:
         LinkSubresource,
         TextTrack,
         Shader,
-        ImportResource
+        ImportResource,
+        NumberOfTypes = 13,

[abarth-chromium, 2013/09/16 21:04:35]

There's no need to assign a specific value to this name.  The compiler will give
it the right value automatically.

[Philippe, 2013/09/17 08:37:50]

Yeah, I had kept this one to make it clear that this value has a special meaning
and should be kept last. Given that the values are not sorted it would be easy
for someone not to pay attention to this value and add a new value at the end
after this one. But I removed it anyway.

     };
 
     enum Status {