Saving an extra backing store if the contents fill the entire border rect.

Source/core/rendering/CompositedLayerMapping.cpp

Index: Source/core/rendering/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/CompositedLayerMapping.cpp b/Source/core/rendering/CompositedLayerMapping.cpp
index c94331dbb80fb1a8ac0984eaf5588afd697a6d29..10b6ec8172a0bab211213e6cd3052678e7c0678a 100644
--- a/Source/core/rendering/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/CompositedLayerMapping.cpp
@@ -1609,11 +1609,16 @@ bool CompositedLayerMapping::containsPaintedContent(bool isSimpleContainer) cons
     if (isSimpleContainer || paintsIntoCompositedAncestor() || m_artificiallyInflatedBounds || m_owningLayer->isReflection())
         return false;
 
+    if (renderer()->isCanvas() || renderer()->isVideo() || renderer()->isImage()) {
+        if (isAcceleratedCanvas(renderer())) {
+            if (contentsBox() == toRenderBox(renderer())->borderBoxRect())
+                return true;
+        }
+    }
+

[vivekg, 2014/02/18 12:30:39]

How about reordering like here? 

if (renderer()->isVideo() || 
    renderer()->isImage() || 
    (renderer()->isCanvas() && isAcceleratedCanvas(renderer()))) {
    if (contentsBox() == toRenderBox(renderer())->borderBoxRect())
        return true;
}

     if (isDirectlyCompositedImage())
         return false;
 
-    // FIXME: we could optimize cases where the image, video or canvas is known to fill the border box entirely,
-    // and set background color on the layer in that case, instead of allocating backing store and painting.
     if (renderer()->isVideo() && toRenderVideo(renderer())->shouldDisplayVideo())
         return m_owningLayer->hasBoxDecorationsOrBackground();