Plumb CompositingReason from Blink to compositor.

Source/core/rendering/RenderLayerBacking.cpp

Index: Source/core/rendering/RenderLayerBacking.cpp
diff --git a/Source/core/rendering/RenderLayerBacking.cpp b/Source/core/rendering/RenderLayerBacking.cpp
index 7423b135b4a0d022eb141a62b2df1e6a896f805d..718b8b8480bac183fc9a54e0eb244a1eebf41044 100644
--- a/Source/core/rendering/RenderLayerBacking.cpp
+++ b/Source/core/rendering/RenderLayerBacking.cpp
@@ -50,6 +50,7 @@
 #include "core/rendering/RenderEmbeddedObject.h"
 #include "core/rendering/RenderIFrame.h"
 #include "core/rendering/RenderImage.h"
+#include "core/rendering/RenderLayer.h" // FIXME - only needed for CompositingReasons, see if we can move that to a better home.

[jamesr, 2013/05/17 00:10:20]

RenderLayerBacking reaches into RenderLayers all over the place, I think this
#include is fine. I'd remove the FIXME

 #include "core/rendering/RenderLayerCompositor.h"
 #include "core/rendering/RenderVideo.h"
 #include "core/rendering/RenderView.h"
@@ -346,6 +347,45 @@ void RenderLayerBacking::updateAfterWidgetResize()
     }
 }
 
+void RenderLayerBacking::updateCompositingReasons()
+{
+    m_graphicsLayer->setCompositingReasons(m_owningLayer->compositingReasons());
+
+    if (m_ancestorClippingLayer)
+        m_ancestorClippingLayer->setCompositingReasons(CompositingReasonLayerForClip);
+
+    if (m_contentsContainmentLayer)
+        m_contentsContainmentLayer->setCompositingReasons(CompositingReasonNone);
+
+    if (m_foregroundLayer)
+        m_foregroundLayer->setCompositingReasons(CompositingReasonsLayerForForeground);
+
+    if (m_backgroundLayer)
+        m_backgroundLayer->setCompositingReasons(CompositingReasonNone);
+
+    if (m_childContainmentLayer)
+        m_childContainmentLayer->setCompositingReasons(CompositingReasonNone);
+
+    if (m_maskLayer)
+        m_maskLayer->setCompositingReasons(CompositingReasonNone);
+
+    if (m_layerForHorizontalScrollbar)
+        m_layerForHorizontalScrollbar->setCompositingReasons(CompositingReasonLayerForScrollbar);
+
+    if (m_layerForVerticalScrollbar)
+        m_layerForVerticalScrollbar->setCompositingReasons(CompositingReasonLayerForScrollbar);
+
+    if (m_layerForScrollCorner)
+        m_layerForScrollCorner->setCompositingReasons(CompositingReasonLayerForScrollbar);
+
+    // Is this the correct reason?
+    if (m_scrollingLayer)
+        m_scrollingLayer->setCompositingReasons(CompositingReasonLayerForDescendantClip);
+
+    if (m_scrollingContentsLayer)
+        m_scrollingContentsLayer->setCompositingReasons(CompositingReasonsLayerForScrollingContainer);
+}
+
 void RenderLayerBacking::updateAfterLayout(UpdateAfterLayoutFlags flags)
 {
     RenderLayerCompositor* layerCompositor = compositor();
@@ -718,6 +758,8 @@ void RenderLayerBacking::updateGraphicsLayerGeometry()
     updateDrawsContent(isSimpleContainer);
     updateAfterWidgetResize();
     registerScrollingLayers();
+
+    updateCompositingReasons();
 }
 
 void RenderLayerBacking::registerScrollingLayers()