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

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

 // 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.
 
 #include "core/css/resolver/StyleRuleUsageTracker.h"
 
 #include "core/css/StyleRule.h"
 
 namespace blink {
 
-bool StyleRuleUsageTracker::contains(StyleRule* rule) const {
-  return m_ruleList.contains(rule);
+HeapVector<Member<StyleRule>> StyleRuleUsageTracker::takeDelta() {
+  HeapVector<Member<StyleRule>> result;
+  result.swap(m_ruleListDelta);
+  return result;
 }
 
 DEFINE_TRACE(StyleRuleUsageTracker) {
   visitor->trace(m_ruleList);
+  visitor->trace(m_ruleListDelta);
 }
 
 }  // namespace blink