DevTools: add support for polling for coverage data in CSS agent

third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h

Index: third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h b/third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h
index 1f76f55624680856b43774dec30bec17595c764f..da4b0e9832bfa00cb29ec447fdd92916b7cad67b 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h
+++ b/third_party/WebKit/Source/core/css/resolver/StyleRuleUsageTracker.h
@@ -13,14 +13,17 @@ class StyleRule;
 
 class StyleRuleUsageTracker : public GarbageCollected<StyleRuleUsageTracker> {
  public:
-  void track(StyleRule* rule) { m_ruleList.insert(rule); }
-
-  bool contains(StyleRule*) const;
+  void track(StyleRule* rule) {
+    if (m_ruleList.insert(rule).isNewEntry)
+      m_ruleListDelta.push_back(rule);
+  }
+  HeapVector<Member<StyleRule>> takeDelta();
 
   DECLARE_TRACE();
 
  private:
   HeapHashSet<Member<StyleRule>> m_ruleList;
+  HeapVector<Member<StyleRule>> m_ruleListDelta;
 };
 
 }  // namespace blink