Don't create render passes for transparent images.

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index ca7d45050f85c0857e5f88a4def5818ef1b5374e..6ccca2cb5a16db4846959b9de6296afd53b68a77 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -259,7 +259,7 @@ static bool subtreeShouldRenderToSeparateSurface(LayerType* layer, bool axisAlig
         return true;
 
     // If the layer has opacity != 1 and does not have a preserves-3d transform style.
-    if (layer->opacity() != 1 && !layer->preserves3D() && descendantDrawsContent)
+    if (layer->opacity() != 1 && !layer->preserves3D() && layer->drawsContent() && descendantDrawsContent)
         return true;
 
     return false;
@@ -484,10 +484,10 @@ static void calculateDrawTransformsInternal(LayerType* layer, const WebTransform
 
     gfx::Rect clipRectForSubtree;
     bool subtreeShouldBeClipped = false;
-    
+
     float drawOpacity = layer->opacity();
     bool drawOpacityIsAnimating = layer->opacityIsAnimating();
-    if (layer->parent() && layer->parent()->preserves3D()) {
+    if (layer->parent()) {

[danakj, 2012/11/26 18:35:26]

This should be replaced with a check for something else.. like:

if (layer->parent() && !layer->parent()->renderSurface())

When the parent owns a surface, the opacity will be applied at draw time of the
surface, and we don't need to inherit it.

[shawnsingh, 2012/11/26 18:52:57]

danakj@ is this true?  we're using the parent layer's drawOpacity, which already
accounts for whether or not its own opacity was re-parented by its
renderSurface.   I think the additional if-statement portion you're proposing
would be a no-op, wouldn't it?   

If what I said is correct, I'm still OK with whichever way seems to make the
semantics more obvious.  But a comment is definitely needed here, and maybe that
makes it clear enough the situation without the additional if-statement.

[danakj, 2012/11/26 18:57:34]

Oh that is true.

I thought we had the preserves3D here because then you know the parent didn't
own a surface. This check is from so long ago and has just been bumped around
over time.

Since we use the drawOpacity, this change seems fine as is to me also.

         drawOpacity *= layer->parent()->drawOpacity();
         drawOpacityIsAnimating |= layer->parent()->drawOpacityIsAnimating();
     }