Make ViewportScrollCallback an interface and add child and root classes

third_party/WebKit/Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 1992 matching lines @@
             'page/PopupOpeningObserver.h',
             'page/PrintContext.cpp',
             'page/ScopedPageLoadDeferrer.cpp',
             'page/ScopedPageLoadDeferrer.h',
             'page/SpatialNavigation.cpp',
             'page/TouchAdjustment.cpp',
             'page/TouchAdjustment.h',
             'page/TouchDisambiguation.cpp',
             'page/TouchDisambiguation.h',
             'page/WindowFeatures.cpp',
+            'page/scrolling/ChildViewportScrollCallback.cpp',
+            'page/scrolling/ChildViewportScrollCallback.h',
             'page/scrolling/OverscrollController.cpp',
             'page/scrolling/OverscrollController.h',
             'page/scrolling/RootScrollerController.cpp',
             'page/scrolling/RootScrollerController.h',
+            'page/scrolling/RootViewportScrollCallback.cpp',
+            'page/scrolling/RootViewportScrollCallback.h',
             'page/scrolling/ScrollingCoordinator.cpp',
             'page/scrolling/ScrollingCoordinator.h',
             'page/scrolling/SnapCoordinator.cpp',
             'page/scrolling/SnapCoordinator.h',
             'page/scrolling/ScrollCustomizationCallbacks.cpp',
             'page/scrolling/ScrollCustomizationCallbacks.h',
             'page/scrolling/ScrollState.cpp',
             'page/scrolling/ScrollState.h',
             'page/scrolling/ScrollStateCallback.cpp',
             'page/scrolling/ScrollStateCallback.h',
             'page/scrolling/StickyPositionScrollingConstraints.cpp',
             'page/scrolling/StickyPositionScrollingConstraints.h',
-            'page/scrolling/ViewportScrollCallback.cpp',
             'page/scrolling/ViewportScrollCallback.h',
             'paint/BackgroundImageGeometry.cpp',
             'paint/BackgroundImageGeometry.h',
             'paint/BlockFlowPainter.cpp',
             'paint/BlockFlowPainter.h',
             'paint/BlockPainter.cpp',
             'paint/BlockPainter.h',
             'paint/BoxBorderPainter.cpp',
             'paint/BoxBorderPainter.h',
             'paint/BoxClipper.cpp',
@@ skipping 2154 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }