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

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef StyleRuleUsageTracker_h
 #define StyleRuleUsageTracker_h
 
 #include "core/css/CSSStyleRule.h"
 
 namespace blink {
 
 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
 
 #endif  // StyleRuleUsageTracker_h