Adding counters for index accessors

Bug: 
Change-Id: I272123ce602510eef102fbe58516c5dd37c23af0
Reviewed-on: https://chromium-review.googlesource.com/780680
Commit-Queue: Natalie Silvanovich <natashenka@google.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518380}

third_party/WebKit/Source/bindings/core/v8/UseCounterCallback.cpp

@@ -142,6 +142,9 @@ void UseCounterCallback(v8::Isolate* isolate,
     case v8::Isolate::kErrorStackTraceLimit:
       blink_feature = WebFeature::kV8ErrorStackTraceLimit;
       break;
+    case v8::Isolate::kIndexAccessor:
+      blink_feature = WebFeature::kV8IndexAccessor;
+      break;
     default:
       // This can happen if V8 has added counters that this version of Blink
       // does not know about. It's harmless.

third_party/WebKit/public/platform/web_feature.mojom

@@ -1760,6 +1760,7 @@ enum WebFeature {
   kHTMLFrameSetElementNonNullAnonymousNamedGetter = 2235,
   kCSPWithUnsafeEval = 2236,
   kWebAssemblyInstantiation = 2237,
+  kV8IndexAccessor = 2238,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -16994,6 +16994,7 @@ Called by update_net_error_codes.py.-->
   <int value="2235" label="HTMLFrameSetElementNonNullAnonymousNamedGetter"/>
   <int value="2236" label="CSPWithUnsafeEval"/>
   <int value="2237" label="WebAssemblyInstantiation"/>
+  <int value="2238" label="V8IndexAccessor"/>
 </enum>
 
 <enum name="FeedbackSource">