Add histograms to ResourceFetcher to evaluate memory cache hit rate

Source/core/fetch/ResourceFetcher.h

Index: Source/core/fetch/ResourceFetcher.h
diff --git a/Source/core/fetch/ResourceFetcher.h b/Source/core/fetch/ResourceFetcher.h
index 5afdc26e9c0cdeb13255ba1bbb5f39ff52da0f0e..97cccaffb130314928f0df2c126f3e5218bbb27a 100644
--- a/Source/core/fetch/ResourceFetcher.h
+++ b/Source/core/fetch/ResourceFetcher.h
@@ -226,6 +226,11 @@ private:
     OwnPtr<ResourceLoaderSet> m_loaders;
     OwnPtr<ResourceLoaderSet> m_multipartLoaders;
 
+    // Used in hit rate histograms.
+    int m_useCount;
+    int m_revalidateCount;
+    int m_loadCount;
+
     // 29 bits left
     bool m_autoLoadImages : 1;
     bool m_imagesEnabled : 1;