Let only the root layer paint root background

When the viewport has fixed background, we may create a dedicated
GraphicsLayer (CompositedLayerMapping::m_backgroundLayer) for the
root background. Previously we used PaintLayerPaintingRootBackgroundOnly
and PaintLayerPaintingCompositingForegroundPhase flags when painting
the layer. The second flag was necessary before we refactored viewport
background painting into ViewPainter, because we needed the layout
objects of the document element or the body element to paint the
viewport background.

Now we need only the root layer to paint root background.

BUG=595839
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2294313002
Cr-Commit-Position: refs/heads/master@{#415694}

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

@@ -2464,7 +2464,7 @@ void CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, G
         paintLayerFlags |= PaintLayerPaintingCompositingScrollingPhase;
 
     if (graphicsLayer == m_backgroundLayer.get())
-        paintLayerFlags |= (PaintLayerPaintingRootBackgroundOnly | PaintLayerPaintingCompositingForegroundPhase); // Need PaintLayerPaintingCompositingForegroundPhase to walk child layers.
+        paintLayerFlags |= PaintLayerPaintingRootBackgroundOnly;
     else if (compositor()->fixedRootBackgroundLayer() && m_owningLayer.isRootLayer())
         paintLayerFlags |= PaintLayerPaintingSkipRootBackground;
 

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

@@ -203,7 +203,7 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerContents(GraphicsCon
 
     PaintResult result = FullyPainted;
 
-    if (paintFlags & PaintLayerPaintingRootBackgroundOnly && !m_paintLayer.layoutObject()->isLayoutView() && !m_paintLayer.layoutObject()->isDocumentElement())
+    if (paintFlags & PaintLayerPaintingRootBackgroundOnly && !m_paintLayer.layoutObject()->isLayoutView())
         return result;
 
     if (m_paintLayer.layoutObject()->view()->frame() && m_paintLayer.layoutObject()->view()->frame()->shouldThrottleRendering())
@@ -564,20 +564,13 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintChildren(unsigned childre
     return result;
 }
 
-// FIXME: inline this.
-static bool paintForFixedRootBackground(const PaintLayer* layer, PaintLayerFlags paintFlags)
-{
-    return layer->layoutObject()->isDocumentElement() && (paintFlags & PaintLayerPaintingRootBackgroundOnly);
-}
-
 bool PaintLayerPainter::shouldPaintLayerInSoftwareMode(const GlobalPaintFlags globalPaintFlags, PaintLayerFlags paintFlags)
 {
     DisableCompositingQueryAsserts disabler;
 
     return m_paintLayer.compositingState() == NotComposited
         || (globalPaintFlags & GlobalPaintFlattenCompositingLayers)
-        || ((paintFlags & PaintLayerPaintingReflection) && !m_paintLayer.has3DTransform())
-        || paintForFixedRootBackground(&m_paintLayer, paintFlags);
+        || ((paintFlags & PaintLayerPaintingReflection) && !m_paintLayer.has3DTransform());
 }
 
 void PaintLayerPainter::paintOverflowControlsForFragments(const PaintLayerFragments& layerFragments, GraphicsContext& context, const PaintLayerPaintingInfo& localPaintingInfo, PaintLayerFlags paintFlags)
@@ -677,6 +670,8 @@ void PaintLayerPainter::paintForegroundForFragments(const PaintLayerFragments& l
     GraphicsContext& context, const LayoutRect& transparencyPaintDirtyRect,
     const PaintLayerPaintingInfo& localPaintingInfo, bool selectionOnly, PaintLayerFlags paintFlags)
 {
+    DCHECK(!(paintFlags & PaintLayerPaintingRootBackgroundOnly));
+
     // Optimize clipping for the single fragment case.
     bool shouldClip = localPaintingInfo.clipToDirtyRect && layerFragments.size() == 1 && !layerFragments[0].foregroundRect.isEmpty();
     ClipState clipState = HasNotClipped;