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

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 23 matching lines @@
       'EventDispatchForbiddenScope.cpp',
       'EventDispatchForbiddenScope.h',
       'EventTracer.cpp',
       'EventTracer.h',
       'FileChooser.cpp',
       'FileChooser.h',
       'FileMetadata.cpp',
       'FileMetadata.h',
       'FileSystemType.h',
       'FloatConversion.h',
+      'Histogram.cpp',
+      'Histogram.h',
       'HostWindow.h',
       'JSONParser.cpp',
       'JSONParser.h',
       'JSONValues.cpp',
       'JSONValues.h',
       'JSONValuesForV8.cpp',
       'JSONValuesForV8.h',
       'KeyboardCodes.h',
       'KillRing.h',
       'KillRingNone.cpp',
@@ skipping 1096 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }