Implementing document.setRootScroller API for main thread scrolling.

third_party/WebKit/Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'AudioOutputDeviceClientImpl.cpp',
       'AudioOutputDeviceClientImpl.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
@@ skipping 247 matching lines @@
       'tests/FrameTestHelpers.h',
       'tests/FrameThrottlingTest.cpp',
       'tests/ImeOnFocusTest.cpp',
       'tests/KeyboardTest.cpp',
       'tests/ListenerLeakTest.cpp',
       'tests/LinkSelectionTest.cpp',
       'tests/MHTMLTest.cpp',
       'tests/PrerenderingTest.cpp',
       'tests/ProgrammaticScrollTest.cpp',
       'tests/LayoutGeometryMapTest.cpp',
+      'tests/RootScrollerTest.cpp',
       'tests/ScreenWakeLockTest.cpp',
       'tests/ScrollingCoordinatorTest.cpp',
       'tests/SpinLockTest.cpp',
       'tests/TextFinderTest.cpp',
       'tests/TopControlsTest.cpp',
       'tests/TouchActionTest.cpp',
       'tests/ViewportTest.cpp',
       'tests/VirtualTimeTest.cpp',
       'tests/VisualViewportTest.cpp',
       'tests/WebDocumentTest.cpp',
@@ skipping 21 matching lines @@
       'tests/sim/SimNetwork.h',
       'tests/sim/SimRequest.cpp',
       'tests/sim/SimRequest.h',
       'tests/sim/SimTest.cpp',
       'tests/sim/SimTest.h',
       'tests/sim/SimWebViewClient.cpp',
       'tests/sim/SimWebViewClient.h',
     ],
   },
 }