[RLS] Document's main GraphicsLayer size should not include overflow.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 2485 matching lines @@
     CalculateBoundsOptions options) const {
   if (!isSelfPaintingLayer())
     return LayoutRect();
 
   // FIXME: This could be improved to do a check like
   // hasVisibleNonCompositingDescendantLayers() (bug 92580).
   if (this != &compositedLayer && !hasVisibleContent() &&
       !hasVisibleDescendant())
     return LayoutRect();
 
-  // The root layer is the size of the document, plus any additional area due
-  // to layout viewport being different than initial containing block.
   if (isRootLayer()) {
-    IntRect documentRect = layoutObject().view()->documentRect();
-
-    if (FrameView* frameView = layoutObject().document().view()) {
-      documentRect.unite(IntRect(IntPoint(), frameView->visibleContentSize()));
-    }
-
-    return LayoutRect(documentRect);
+    // In root layer scrolling mode, the main GraphicsLayer is the size of the
+    // layout viewport. In non-RLS mode, it is the union of the layout viewport
+    // and the document's layout overflow rect.
+    IntRect result = IntRect();
+    if (FrameView* frameView = layoutObject().frameView())
+      result = IntRect(IntPoint(), frameView->visibleContentSize());
+    if (!RuntimeEnabledFeatures::rootLayerScrollingEnabled())
+      result.unite(layoutObject().view()->documentRect());
+    return LayoutRect(result);
   }
 
   // The layer created for the LayoutFlowThread is just a helper for painting
   // and hit-testing, and should not contribute to the bounding box. The
   // LayoutMultiColumnSets will contribute the correct size for the layout
   // content of the multicol container.
   if (layoutObject().isLayoutFlowThread())
     return LayoutRect();
 
   const_cast<PaintLayer*>(this)->stackingNode()->updateLayerListsIfNeeded();
@@ skipping 701 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif