Add histograms to ResourceFetcher to measure memory cache hit rate.

Source/core/fetch/Resource.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 52 matching lines @@
         CSSStyleSheet,
         Script,
         Font,
         Raw,
         SVGDocument,
         XSLStyleSheet,
         LinkPrefetch,
         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 {
         Unknown, // let cache decide what to do with it
         Pending, // only partially loaded
         Cached, // regular case
         LoadError,
         DecodeError
     };
 
@@ skipping 269 matching lines @@
     // If this field is non-null, the resource has a proxy for checking whether it is still up to date (see m_resourceToRevalidate).
     Resource* m_proxyResource;
 
     // These handles will need to be updated to point to the m_resourceToRevalidate in case we get 304 response.
     HashSet<ResourcePtrBase*> m_handlesToRevalidate;
 };
 
 }
 
 #endif