Add PlatformFrameView class.

third_party/WebKit/Source/platform/PlatformFrameView.h

+// Copyright 2017 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 PlatformFrameView_h
+#define PlatformFrameView_h
+
+#include "platform/PlatformExport.h"
+
+namespace blink {
+
+// PlatformFrameView is a base class for core/frame/FrameView. PlatformFrameView
+// is needed to let the platform/ layer access functionalities of FrameView.
+class PLATFORM_EXPORT PlatformFrameView {
+ public:
+  PlatformFrameView() {}
+  virtual ~PlatformFrameView() {}
+
+  virtual bool IsFrameView() const { return false; }
+};
+
+}  // namespace blink
+
+#endif  // PlatformFrameView_h