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..686752bbcd8d5265f422198240d88bb05d90dbad 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); }
+  using RuleListByStyleSheet = HeapHashMap<Member<const CSSStyleSheet>,
+                                           HeapVector<Member<const StyleRule>>>;
 
-  bool contains(StyleRule*) const;
+  void track(const StyleRule*, const CSSStyleSheet*);
+  RuleListByStyleSheet takeDelta();
 
   DECLARE_TRACE();
 
  private:
-  HeapHashSet<Member<StyleRule>> m_ruleList;
+  HeapHashSet<Member<const StyleRule>> m_usedRules;
+  RuleListByStyleSheet m_usedRulesDelta;
 };
 
 }  // namespace blink