Fix PaintPropertyTreeBuilder for root layer scrolling.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
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@@ skipping 1110 matching lines @@
             bottomLayer = layer;
         }
     };
 
     updateBottomLayer(m_childTransformLayer.get());
     updateBottomLayer(m_childContainmentLayer.get());
     updateBottomLayer(m_scrollingLayer.get());
 
     // Now constructing the subtree for the overflow controls.
     bottomLayer = m_graphicsLayer.get();
-    if (m_isMainFrameLayoutViewLayer)
+    if (m_isMainFrameLayoutViewLayer && !RuntimeEnabledFeatures::slimmingPaintV2Enabled())

[skobes, 2016/08/12 22:26:46]

The purpose of this check is to pull the scrollbars out of the page scale layer
so they don't grow and shrink with pinch zoom.  Don't we want to keep that
behavior in slimming paint v2?

[szager1, 2016/08/12 22:44:31]

As I understand it -- and this was all explained to me by pdr@ -- all of this
GraphicsLayer stuff is basically obsolete when SPv2 is enabled.  There's only
one graphics layer, and the compositor does the rest.  So really, most or all of
CompositedLayerMapping will go away.

The reason I put this here is because it prevents a crash, but with SPv2 this
probably all pointless and obsolete anyway.

[pdr., 2016/08/13 02:25:37]

We do want to maintain the pinch/zoom behavior, but using transform nodes
instead of graphics layers. I don't think anyone has thought about how either
pinch/zoom or page scale will hook up in spv2 though. For pinch/zoom, could we
add a special-case transform property node on layoutView that is before the
transform node? We can't reuse the layoutView's transform property since it can
get used (e.g., http://jsbin.com/zabufa). Any way we go, we'll need special
logic to use one transform node for the scrollbars but another for the contents
:/

SPV2 will indeed get rid of GraphicsLayer altogether, deferring to cc for what
gets layerized.

         bottomLayer = layoutObject()->frame()->page()->frameHost().visualViewport().containerLayer();
     updateBottomLayer(m_overflowControlsAncestorClippingLayer.get());
     updateBottomLayer(m_overflowControlsHostLayer.get());
     if (m_layerForHorizontalScrollbar)
         m_overflowControlsHostLayer->addChild(m_layerForHorizontalScrollbar.get());
     if (m_layerForVerticalScrollbar)
         m_overflowControlsHostLayer->addChild(m_layerForVerticalScrollbar.get());
     if (m_layerForScrollCorner)
         m_overflowControlsHostLayer->addChild(m_layerForScrollCorner.get());
 
@@ skipping 1518 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink