Add PlatformFrameView class.

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index a1d552cb17c1e9e75c3464d24302ef7bfb57d68e..92309bd55b61fff3d25281fc6f741cb7d41d4331 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -42,6 +42,7 @@
 #include "core/paint/ScrollbarManager.h"
 #include "core/plugins/PluginView.h"
 #include "platform/FrameViewBase.h"
+#include "platform/PlatformFrameView.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/animation/CompositorAnimationHost.h"
 #include "platform/animation/CompositorAnimationTimeline.h"
@@ -103,6 +104,7 @@ typedef unsigned long long DOMTimeStamp;
 
 class CORE_EXPORT FrameView final
     : public GarbageCollectedFinalized<FrameView>,
+      public PlatformFrameView,
       public FrameViewBase,
       public FrameOrPlugin,
       public PaintInvalidationCapableScrollableArea {
@@ -1267,6 +1269,11 @@ inline void FrameView::IncrementVisuallyNonEmptyPixelCount(
     SetIsVisuallyNonEmpty();
 }
 
+DEFINE_TYPE_CASTS(FrameView,
+                  PlatformFrameView,
+                  platform_frame_view,
+                  platform_frame_view->IsFrameView(),
+                  platform_frame_view.IsFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   FrameViewBase,
                   frameViewBase,