Create base class for common functionality of Web{Local,Remote}Frame

third_party/WebKit/Source/web/WebFrameImplBase.h

Index: third_party/WebKit/Source/web/WebFrameImplBase.h
diff --git a/third_party/WebKit/Source/web/WebFrameImplBase.h b/third_party/WebKit/Source/web/WebFrameImplBase.h
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+// 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 WebFrameImplBase_h
+#define WebFrameImplBase_h
+
+#include "platform/heap/Handle.h"
+#include "wtf/text/AtomicString.h"
+
+namespace blink {
+
+class Frame;
+class FrameHost;
+class FrameOwner;
+
+// WebFrameImplBase exists to avoid the diamond inheritance problem:
+// - The public interfaces WebLocalFrame/WebRemoteFrame extend WebFrame.
+// - WebLocalFrameImpl implements WebLocalFrame and WebRemoteFrameImpl
+//   implements WebRemoteFrame.
+// - The private implementations should share some functionality, but cannot
+//   inherit from a common base class inheriting WebFrame. This would result in
+//   WebFrame beind inherited from two different base classes.
+//
+// To get around this, only the private implementations have WebFrameImplBase as
+// a base class. WebFrame exposes a virtual accessor to retrieve the underlying
+// implementation as an instance of the base class, but has no inheritance
+// relationship with it. The cost is a virtual indirection, but this is nicer
+// than the previous manual dispatch emulating real virtual dispatch.
+class WebFrameImplBase : public RefCountedWillBeGarbageCollectedFinalized<WebFrameImplBase> {
+public:
+    virtual ~WebFrameImplBase();
+
+    virtual void initializeCoreFrame(FrameHost*, FrameOwner*, const AtomicString& name, const AtomicString& fallbackName) = 0;
+    // TODO(dcheng): Rename this to coreFrame()? This probably also shouldn't be const...
+    virtual Frame* frame() const = 0;
+
+    DECLARE_VIRTUAL_TRACE();
+};
+
+} // namespace blink
+
+#endif // WebFrameImplBase_h