[root layer scrolls] Repaint scrolling on bg color override change

This patch ensures the scrolling contents layer is repainted when
the background color is changed with the inspector protocol.

Bug: 781428
Change-Id: Ia7ba3e7acdf55bf557e42cd9fab62d08f2ce3a0c
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_root_layer_scrolls
Reviewed-on: https://chromium-review.googlesource.com/805006
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521996}

third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls

@@ -22,7 +22,6 @@ crbug.com/417782 [ Linux Mac ] fast/spatial-navigation/snav-iframe-with-offscree
 crbug.com/417782 [ Linux Mac ] fast/spatial-navigation/snav-media-elements.html [ Failure ]
 crbug.com/417782 [ Linux ] fast/spatial-navigation/snav-unit-overflow-and-scroll-in-direction.html [ Failure ]
 crbug.com/417782 html/dialog/form-method-dialog.html [ Crash ]
-crbug.com/417782 http/tests/devtools/device-mode/default-background-color.html [ Failure ]
 crbug.com/417782 http/tests/devtools/tracing/scroll-invalidations.js [ Failure ]
 crbug.com/417782 http/tests/devtools/tracing/timeline-paint/layer-tree.js [ Failure ]
 crbug.com/417782 http/tests/devtools/tracing/timeline-paint/timeline-paint-with-layout-invalidations-on-deleted-node.js [ Failure ]

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

@@ -2444,6 +2444,8 @@ void LocalFrameView::SetBaseBackgroundColor(const Color& background_color) {
     composited_layer_mapping->UpdateContentsOpaque();
     if (composited_layer_mapping->MainGraphicsLayer())
       composited_layer_mapping->MainGraphicsLayer()->SetNeedsDisplay();
+    if (composited_layer_mapping->ScrollingContentsLayer())
+      composited_layer_mapping->ScrollingContentsLayer()->SetNeedsDisplay();
   }
   RecalculateScrollbarOverlayColorTheme(DocumentBackgroundColor());