Draw main frame custom scrollbars in root layer scrolling mode.

third_party/WebKit/Source/core/frame/RootFrameViewport.cpp

Index: third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
diff --git a/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp b/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
index fee7310bad65be23c40f23f626111291570a1801..3077ec5d8257e7059731fc138da82316843c3bf4 100644
--- a/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
+++ b/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
@@ -5,7 +5,11 @@
 #include "core/frame/RootFrameViewport.h"
 
 #include "core/frame/FrameView.h"
+#include "core/frame/LocalFrame.h"
 #include "core/layout/ScrollAlignment.h"
+#include "core/layout/compositing/CompositedLayerMapping.h"
+#include "core/page/Page.h"
+#include "core/paint/PaintLayer.h"
 #include "platform/geometry/DoubleRect.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/LayoutRect.h"
@@ -86,6 +90,35 @@ LayoutBox* RootFrameViewport::layoutBox() const
     return layoutViewport().layoutBox();
 }
 
+LocalFrame* RootFrameViewport::mainFrame() const
+{
+    if (!layoutBox())
+        return 0;
+
+    LocalFrame* frame = layoutBox()->frame();
+    if (!frame && !frame->page())
+        return 0;
+
+    return frame->page()->mainFrame() && frame->page()->mainFrame()->isLocalFrame() ? frame->page()->deprecatedLocalMainFrame() : 0;
+}
+
+CompositedLayerMapping* RootFrameViewport::compositedLayerMapping() const
+{
+
+    LocalFrame* frame = mainFrame();
+
+    if (!frame || !frame->view())
+        return 0;
+
+    if (frame && frame->view()) {
+        FrameView* view = frame->view();
+        if (!view->layoutViewItem().isNull() && view->layoutViewItem().layer()->hasCompositedLayerMapping())
+            return view->layoutViewItem().layer()->compositedLayerMapping();
+    }
+
+    return 0;
+}
+
 void RootFrameViewport::updateScrollAnimator()
 {
     scrollAnimator().setCurrentPosition(toFloatPoint(scrollOffsetFromScrollAnimators()));
@@ -294,12 +327,50 @@ GraphicsLayer* RootFrameViewport::layerForScrolling() const
 
 GraphicsLayer* RootFrameViewport::layerForHorizontalScrollbar() const
 {
-    return layoutViewport().layerForHorizontalScrollbar();
+    LocalFrame* frame = mainFrame();

[bokan, 2016/09/02 16:01:54]

I don't think you need any of the changes in this file. If root layer scrolls is
enabled, layoutViewport() is the PLSA belonging to the root frame in which case
PaintLayerScrollableArea::layerForHorizontalScrollbar() does what you're doing
here.

If root layer scrolls is disabled, layoutViewport() is the FrameView so
FrameView::layerForHorizontalScrollbar is what you return here as well.

I think you only really need to add the scroll corner method but that can also
delegate to layoutViewport(). Or amy I missing something?

[MuVen, 2016/09/02 16:36:12]

so as i understand its better to retain the old definition 
layoutViewport().layerForHorizontalScrollbar()(correct me if im wrong); here
layoutViewport will vary based on the RLS enabled or disabled. 

if(RLS -> enabled)
   PLSA->layerForHorizontalScrollbar() ; will be invoked
else
   FrameView->layerForHorizontalScrollbar(); will be invoked.

we need to add scrollcorner method delegating to layoutViewport(); as
scrollcorner method is not overridden.

[MuVen, 2016/09/06 10:07:29]

Done.

+
+    if (!frame || !frame->view())
+        return 0;
+
+    if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
+        if (CompositedLayerMapping* m_compositedLayerMapping = compositedLayerMapping()) {
+            return m_compositedLayerMapping->layerForHorizontalScrollbar();
+        }
+    }
+
+    return frame->view()->layerForHorizontalScrollbar();
 }
 
 GraphicsLayer* RootFrameViewport::layerForVerticalScrollbar() const
 {
-    return layoutViewport().layerForVerticalScrollbar();
+    LocalFrame* frame = mainFrame();
+
+    if (!frame || !frame->view())
+        return 0;
+
+    if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
+        if (CompositedLayerMapping* m_compositedLayerMapping = compositedLayerMapping()) {
+            return m_compositedLayerMapping->layerForVerticalScrollbar();
+        }
+    }
+
+    return frame->view()->layerForVerticalScrollbar();
+}
+
+GraphicsLayer* RootFrameViewport::layerForScrollCorner() const
+{
+    LocalFrame* frame = mainFrame();
+
+    if (!frame || !frame->view())
+        return 0;
+
+    if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
+        if (CompositedLayerMapping* m_compositedLayerMapping = compositedLayerMapping()) {
+            return m_compositedLayerMapping->layerForScrollCorner();
+        }
+    }
+
+    return frame->view()->layerForScrollCorner();
 }
 
 ScrollResult RootFrameViewport::userScroll(ScrollGranularity granularity, const FloatSize& delta)