Add use-counters for Client-Hints features

Source/core/fetch/FetchContext.h

Index: Source/core/fetch/FetchContext.h
diff --git a/Source/core/fetch/FetchContext.h b/Source/core/fetch/FetchContext.h
index a5e31cd44fde712e2dc81b38daca178ac1e5bcab..82f9929cb5e88d773f01a1a2d0b0e0674df9b5b4 100644
--- a/Source/core/fetch/FetchContext.h
+++ b/Source/core/fetch/FetchContext.h
@@ -59,10 +59,18 @@ class CORE_EXPORT FetchContext : public NoBaseWillBeGarbageCollectedFinalized<Fe
 public:
     static FetchContext& nullInstance();
 
+    // To avoid a dependency on core/ we redefine fetch based UseCounter features here.
+    enum FetchUseCounterFeature {

[Nate Chapin, 2015/06/16 17:10:28]

It seems unfortunate to have this enum essentially duplicate the
UseCounter::Feature enum. Could we separate UseCounter::Feature from the rest of
UseCounter and only give core/fetch/ access to the enum?

+        ClientHintsDPR,
+        ClientHintsResourceWidth,
+        ClientHintsViewportWidth
+    };
+
     virtual ~FetchContext() { }
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 
     virtual bool isLiveContext() { return false; }
+    virtual void count(FetchUseCounterFeature) { }
 
     virtual void addAdditionalRequestHeaders(ResourceRequest&, FetchResourceType);
     virtual void setFirstPartyForCookies(ResourceRequest&);