Make RootScroller set the outer viewport scroll layer in the compositor

third_party/WebKit/Source/web/WebViewImpl.cpp

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  *
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@@ skipping 1854 matching lines @@
             topControls().layoutHeight() - topControls().contentOffset();
         visualViewport.setTopControlsAdjustment(topControlsViewportAdjustment);
 
         // Since the FrameView is sized to be the visual viewport at minimum
         // scale, its adjustment must also be scaled by the minimum scale.
         view->setTopControlsViewportAdjustment(
             topControlsViewportAdjustment / minimumPageScaleFactor());
     }
 }
 
+void WebViewImpl::didUpdateRootScrollerLayer(WebLayer* rootScrollerLayer)
+{
+    if (!m_layerTreeView)
+        return;
+
+    if (!page()->mainFrame() || !page()->mainFrame()->isLocalFrame())

[dcheng, 2016/06/29 22:34:29]

I'm out of the loop on the root scroller work: is it possible to have a root
scroller in a non-main frame?

[bokan, 2016/06/29 22:37:02]

It is in the sense that an iframe can set document.rootScroller so the machinery
is still there to handle that but it has no visible effect right now so I chose
to simply not tell the compositor about it for remote frames.

+        return;
+
+    m_layerTreeView->setRootScrollerLayer(rootScrollerLayer);
+}
+
 TopControls& WebViewImpl::topControls()
 {
     return page()->frameHost().topControls();
 }
 
 void WebViewImpl::resizeViewWhileAnchored(
     FrameView* view, float topControlsHeight, bool topControlsShrinkLayout)
 {
     DCHECK(mainFrameImpl());
 
@@ skipping 2638 matching lines @@
 {
     // TODO(oshima): Investigate if this should return the ScreenInfo's scale factor rather than
     // page's scale factor, which can be 1 in use-zoom-for-dsf mode.
     if (!page())
         return 1;
 
     return page()->deviceScaleFactor();
 }
 
 } // namespace blink