Implemented RuleSet diff for active stylesheets.

third_party/WebKit/Source/core/css/ActiveStyleSheets.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 ActiveStyleSheets_h
+#define ActiveStyleSheets_h
+
+#include "core/CoreExport.h"
+#include "platform/heap/HeapAllocator.h"
+
+namespace blink {
+
+class CSSStyleSheet;
+class RuleSet;
+
+using ActiveStyleSheetVector = HeapVector<std::pair<Member<CSSStyleSheet>, Member<RuleSet>>>;
+
+enum ActiveSheetsChange {
+    NoActiveSheetsChanged, // Nothing changed.
+    ActiveSheetsChanged, // Sheets were added and/or inserted.
+    ActiveSheetsAppended // Only additions, and all appended.
+};
+
+CORE_EXPORT ActiveSheetsChange compareActiveStyleSheets(
+    const ActiveStyleSheetVector& oldStyleSheets,
+    const ActiveStyleSheetVector& newStyleSheets,
+    HeapVector<Member<RuleSet>>& changedRuleSets);
+
+} // namespace blink
+
+#endif // ActiveStyleSheets_h