Let backgrounds paint in scrolling contents layer without overflow.

Since r481209, the document creates composited scrolling layers with RLS even if
it doesn't overflow.  But PaintLayer::GetBackgroundPaintLocation didn't get the
memo.

Bug: 741858
Change-Id: I1fe09532eecc2bbd1b71c42e171bf86f4a893382
Reviewed-on: https://chromium-review.googlesource.com/570940Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487557}

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

@@ -2709,7 +2709,9 @@ GraphicsLayer* PaintLayer::GraphicsLayerBacking(const LayoutObject* obj) const {
 BackgroundPaintLocation PaintLayer::GetBackgroundPaintLocation(
     uint32_t* reasons) const {
   BackgroundPaintLocation location;
-  if (!ScrollsOverflow()) {
+  bool has_scrolling_layers =
+      scrollable_area_ && scrollable_area_->NeedsCompositedScrolling();
+  if (!ScrollsOverflow() && !has_scrolling_layers) {
     location = kBackgroundPaintInGraphicsLayer;
   } else if (RuntimeEnabledFeatures::RootLayerScrollingEnabled()) {
     location = GetLayoutObject().GetBackgroundPaintLocation(reasons);

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

@@ -77,8 +77,11 @@ void ViewPainter::PaintBoxDecorationBackground(const PaintInfo& paint_info) {
         static_cast<const DisplayItemClient*>(layout_view_.Layer()
                                                   ->GetCompositedLayerMapping()
                                                   ->ScrollingContentsLayer());
-    scroll_recorder.emplace(paint_info.context, layout_view_, paint_info.phase,
-                            layout_view_.ScrolledContentOffset());
+    if (!layout_view_.ScrolledContentOffset().IsZero()) {
+      scroll_recorder.emplace(paint_info.context, layout_view_,
+                              paint_info.phase,
+                              layout_view_.ScrolledContentOffset());
+    }
   }
 
   if (DrawingRecorder::UseCachedDrawingIfPossible(