Support caching histograms so that a lookup isn't done in each iteration.

third_party/WebKit/public/platform/Platform.h

Index: third_party/WebKit/public/platform/Platform.h
diff --git a/third_party/WebKit/public/platform/Platform.h b/third_party/WebKit/public/platform/Platform.h
index 885a348626efa382279fe6fb63afd55484c1ea4e..3ea0f624a27be3faab66ef2c3b0a92790a60e931 100644
--- a/third_party/WebKit/public/platform/Platform.h
+++ b/third_party/WebKit/public/platform/Platform.h
@@ -448,13 +448,27 @@ public:
     // Get a pointer to testing support interfaces. Will not be available in production builds.
     virtual WebUnitTestSupport* unitTestSupport() { return nullptr; }
 
+    typedef void* HistogramCacheSlot;
+
     // Callbacks for reporting histogram data.
     // CustomCounts histogram has exponential bucket sizes, so that min=1, max=1000000, bucketCount=50 would do.
-    virtual void histogramCustomCounts(const char* name, int sample, int min, int max, int bucketCount) { }
+    // |cacheSlot| is intended to be a thread safe pointer to an entry that
+    // the histogram code can store a lookup value in. Note that |name|
+    // MUST be the same as a previous call if providing a |cacheSlot| that
+    // was used in a previous call.
+    virtual void histogramCustomCounts(const char* name, int sample, int min, int max, int bucketCount, HistogramCacheSlot* cacheSlot = 0) {}
     // Enumeration histogram buckets are linear, boundaryValue should be larger than any possible sample value.
-    virtual void histogramEnumeration(const char* name, int sample, int boundaryValue) { }
+    // |cacheSlot| is intended to be a thread safe pointer to an entry that
+    // the histogram code can store a lookup value in. Note that |name|
+    // MUST be the same as a previous call if providing a |cacheSlot| that
+    // was used in a previous call.
+    virtual void histogramEnumeration(const char* name, int sample, int boundaryValue, HistogramCacheSlot* cacheSlot = 0) {}
     // Unlike enumeration histograms, sparse histograms only allocate memory for non-empty buckets.
-    virtual void histogramSparse(const char* name, int sample) { }
+    // |cacheSlot| is intended to be a thread safe pointer to an entry that
+    // the histogram code can store a lookup value in. Note that |name|
+    // MUST be the same as a previous call if providing a |cacheSlot| that
+    // was used in a previous call.
+    virtual void histogramSparse(const char* name, int sample, HistogramCacheSlot* cacheSlot = 0) {}
 
     // Record to a RAPPOR privacy-preserving metric, see: https://www.chromium.org/developers/design-documents/rappor.
     // recordRappor records a sample string, while recordRapporURL records the domain and registry of a url.