UseCounter: Introduce UseCounter::Observer for layout tests

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

Index: third_party/WebKit/Source/core/frame/UseCounter.cpp
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.cpp b/third_party/WebKit/Source/core/frame/UseCounter.cpp
index f424c1430b58935699917abd1a4a054bb88f957c..79272a3075bb1af7a4b11567b0cf8e1fbf853488 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.cpp
+++ b/third_party/WebKit/Source/core/frame/UseCounter.cpp
@@ -1125,6 +1125,7 @@ void UseCounter::recordMeasurement(Feature feature) {
       TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("blink.feature_usage"),
                    "FeatureFirstUsed", "feature", feature);
       featuresHistogram().count(feature);
+      notifyFeatureCounted(feature);
     }
     m_featuresRecorded.quickSet(feature);
   }
@@ -1143,6 +1144,10 @@ bool UseCounter::hasRecordedMeasurement(Feature feature) const {
   return m_featuresRecorded.quickGet(feature);
 }
 
+DEFINE_TRACE(UseCounter) {
+  visitor->trace(m_observers);
+}
+
 void UseCounter::didCommitLoad(KURL url) {
   m_legacyCounter.updateMeasurements();
 
@@ -1193,6 +1198,11 @@ bool UseCounter::isCounted(CSSPropertyID unresolvedProperty) {
   return m_CSSRecorded.quickGet(unresolvedProperty);
 }
 
+void UseCounter::addObserver(Observer* observer) {
+  DCHECK(!m_observers.contains(observer));
+  m_observers.insert(observer);
+}
+
 bool UseCounter::isCounted(Document& document, const String& string) {
   Frame* frame = document.frame();
   if (!frame)
@@ -1249,6 +1259,17 @@ void UseCounter::count(Feature feature) {
   recordMeasurement(feature);
 }
 
+void UseCounter::notifyFeatureCounted(Feature feature) {
+  DCHECK(!m_muteCount);
+  DCHECK(!m_disableReporting);
+  HeapHashSet<Member<Observer>> toBeRemoved;
+  for (auto observer : m_observers) {
+    if (observer->onCountFeature(feature))
+      toBeRemoved.insert(observer);
+  }
+  m_observers.removeAll(toBeRemoved);
+}
+
 EnumerationHistogram& UseCounter::featuresHistogram() const {
   // Every SVGImage has it's own Page instance, and multiple web pages can
   // share the usage of a single SVGImage.  Ideally perhaps we'd delegate