Make ViewportScrollCallback an interface and add child and root classes

third_party/WebKit/Source/core/page/scrolling/ChildViewportScrollCallback.h

Index: third_party/WebKit/Source/core/page/scrolling/ChildViewportScrollCallback.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/ChildViewportScrollCallback.h b/third_party/WebKit/Source/core/page/scrolling/ChildViewportScrollCallback.h
new file mode 100644
index 0000000000000000000000000000000000000000..bab9d870382a8b6c29dfdd09d3283c60923d9eff
--- /dev/null
+++ b/third_party/WebKit/Source/core/page/scrolling/ChildViewportScrollCallback.h
@@ -0,0 +1,42 @@
+// 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 ChildViewportScrollCallback_h
+#define ChildViewportScrollCallback_h
+
+#include "core/page/scrolling/ViewportScrollCallback.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class ScrollableArea;
+class ScrollState;
+
+// The ViewportScrollCallback used by non-root Frames, i.e. iframes. This is
+// essentially a no-op implementation that simply scrolls the frame since
+// iframes don't have any additional scrolling actions, unlike the root frame
+// which moves the top controls and provides overscroll glow.
+class ChildViewportScrollCallback : public ViewportScrollCallback {
+public:
+    static ChildViewportScrollCallback* create()
+    {
+        return new ChildViewportScrollCallback();
+    }
+
+    virtual ~ChildViewportScrollCallback();
+
+    void handleEvent(ScrollState*) override;
+    void setScroller(ScrollableArea*) override;
+
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    ChildViewportScrollCallback();
+
+    WeakMember<ScrollableArea> m_scroller;

[tdresser, 2016/07/06 18:28:18]

Should we move this member to the parent class?

[bokan, 2016/07/06 21:12:04]

It's type will change in RootViewportScrollCallback in the next patch so it
would have to be moved back down.

+};
+
+} // namespace blink
+
+#endif // ChildViewportScrollCallback_h