Add fast-path for propagated variable changes (WIP)

third_party/WebKit/Source/core/style/StyleInheritedVariables.h

 // Copyright 2015 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 StyleInheritedVariables_h
 #define StyleInheritedVariables_h
 
 #include "core/css/CSSValue.h"
 #include "core/css/CSSVariableData.h"
 #include "wtf/Forward.h"
@@ skipping 33 matching lines @@
   // Note that this method is slow as a new map is constructed.
   std::unique_ptr<HashMap<AtomicString, RefPtr<CSSVariableData>>> getVariables()
       const;
 
  private:
   StyleInheritedVariables() : m_root(nullptr) {}
   StyleInheritedVariables(StyleInheritedVariables& other);
 
   friend class CSSVariableResolver;
 
+  // Variables registered in this node.
   HashMap<AtomicString, RefPtr<CSSVariableData>> m_data;
+
+  // Custom property values, registered with registerProperty().
   HashMap<AtomicString, Persistent<CSSValue>> m_registeredData;
+
+  // Parent variable values to inherit from.
   RefPtr<StyleInheritedVariables> m_root;
 };
 
 }  // namespace blink
 
 #endif  // StyleInheritedVariables_h