UseCounter: Introduce UseCounter::Observer for layout tests

third_party/WebKit/Source/core/frame/UseCounter.h

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index b78eab5f47bd63b7510a6bc5bd40ef2df1c29e66..a874950eaa1b02c8ea691e48eaa20c171ecde7fe 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -26,14 +26,16 @@
 #ifndef UseCounter_h
 #define UseCounter_h
 
+#include <v8.h>
 #include "core/CSSPropertyNames.h"
 #include "core/CoreExport.h"
 #include "core/css/parser/CSSParserMode.h"
+#include "platform/heap/GarbageCollected.h"
+#include "platform/heap/HeapAllocator.h"
 #include "platform/weborigin/KURL.h"
 #include "wtf/BitVector.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/text/WTFString.h"
-#include <v8.h>
 
 namespace blink {
 
@@ -1464,6 +1466,15 @@ class CORE_EXPORT UseCounter {
     NumberOfFeatures,  // This enum value must be last.
   };
 
+  // An interface to observe UseCounter changes.
+  class Observer : public GarbageCollected<Observer> {
+   public:
+    // Returns true if this observer no longer needs to observe the counter.
+    virtual bool onCountFeature(Feature) = 0;
+
+    DEFINE_INLINE_VIRTUAL_TRACE() {}
+  };
+
   // "count" sets the bit for this feature to 1. Repeated calls are ignored.
   static void count(const Frame*, Feature);
   static void count(const Document&, Feature);
@@ -1484,6 +1495,9 @@ class CORE_EXPORT UseCounter {
   static bool isCounted(Document&, const String&);
   bool isCounted(CSSPropertyID unresolvedProperty);
 
+  // Adds an observer that observes UseCounter changes.
+  static void addObserver(Document&, Observer*);
+
   // Invoked when a new document is loaded into the main frame of the page.
   void didCommitLoad(KURL);
 
@@ -1500,6 +1514,8 @@ class CORE_EXPORT UseCounter {
   // reporting disabled.
   bool hasRecordedMeasurement(Feature) const;
 
+  DECLARE_TRACE();
+
  private:
   EnumerationHistogram& featuresHistogram() const;
   EnumerationHistogram& cssHistogram() const;
@@ -1518,6 +1534,8 @@ class CORE_EXPORT UseCounter {
   BitVector m_featuresRecorded;
   BitVector m_CSSRecorded;
 
+  HeapHashSet<Member<Observer>> m_observers;
+
   // Encapsulates the work to preserve the old "FeatureObserver" histogram with
   // original semantics
   // TODO(rbyers): remove this - http://crbug.com/676837