Draw main frame custom scrollbars in root layer scrolling mode.

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 3b1b7cc80d33f2edf14595b69711e9ab47901362..0134b3d57d3b398f6f78a5f6ac57e39b9c20992a 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -2679,14 +2679,16 @@ void FrameView::synchronizedPaint()
     if (GraphicsLayer* rootGraphicsLayer = view.compositor()->rootGraphicsLayer()) {
         synchronizedPaintRecursively(rootGraphicsLayer);
     }
-    if (GraphicsLayer* layerForHorizontalScrollbar = view.compositor()->layerForHorizontalScrollbar()) {
-        synchronizedPaintRecursively(layerForHorizontalScrollbar);
-    }
-    if (GraphicsLayer* layerForVerticalScrollbar = view.compositor()->layerForVerticalScrollbar()) {
-        synchronizedPaintRecursively(layerForVerticalScrollbar);
-    }
-    if (GraphicsLayer* layerForScrollCorner = view.compositor()->layerForScrollCorner()) {
-        synchronizedPaintRecursively(layerForScrollCorner);
+    if (m_viewportScrollableArea) {

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

This can be a DCHECK since we're the main frame.

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

Need if(m_viewportScrollableArea) check as without which webkit_unit_tests will
crash ex: FrameThrottlingTest.cpp.

[bokan, 2016/09/07 15:00:44]

Interesting, those tests need to be fixed. Please file a bug (cc me) and list
all the tests that break. The problem is that "Main frame doesn't create
RootFrameViewport in some webkit_unit_tests". Please add a TODO here linked to
that bug.

+        if (GraphicsLayer* layerForHorizontalScrollbar = m_viewportScrollableArea->layerForHorizontalScrollbar()) {
+            synchronizedPaintRecursively(layerForHorizontalScrollbar);
+        }
+        if (GraphicsLayer* layerForVerticalScrollbar = m_viewportScrollableArea->layerForVerticalScrollbar()) {
+            synchronizedPaintRecursively(layerForVerticalScrollbar);
+        }
+        if (GraphicsLayer* layerForScrollCorner = m_viewportScrollableArea->layerForScrollCorner()) {
+            synchronizedPaintRecursively(layerForScrollCorner);
+        }
     }
 
     forAllNonThrottledFrameViews([](FrameView& frameView) {