Add flag for drawing layers to screen with Ganesh

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 2ffd61ef4b4243e778a9a0f96bc604c8138fdd08..628572e1602c948d7192f60d074ca9955f9b81e1 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -560,6 +560,11 @@ void LayerImpl::NoteLayerPropertyChangedForDescendants() {
     children_[i]->NoteLayerPropertyChangedForSubtree();
 }
 
+bool LayerImpl::CanDrawDirectlyToBackbuffer() const {

[danakj, 2013/05/15 20:33:16]

I don't like bringing a "composite draw property" into the layer level really.

Could this be a value on draw properties that is computed by LTHCommon?

Or alternatively, a function on DrawProperties? It would then instead need a
state on DrawProps to know that it draws to the root target.

And while we're here, why did you choose to limit this to the root surface only?

[enne (OOO), 2013/05/15 22:34:38]

Moved to DrawProperties.  PTAL.
Limiting to non-1 opacity is because you need to composite all the draw calls in
the picture together before drawing that layer.  This is the picture analogy to
needing to create a render surface for non-1 opacity.

As for the root layer, there'd been some discussion about an initial use for
Ganesh in places where we could get LCD text where we otherwise wouldn't, such
as when you're drawing a layer directly to the backbuffer and not into a render
target.  I just figured we could start there.

What do you think?

+  return render_target() == layer_tree_impl_->root_layer() &&
+      draw_opacity() == 1.f && !draw_opacity_is_animating();
+}
+
 const char* LayerImpl::LayerTypeAsString() const {
   return "Layer";
 }