Allow WebExternalTextureLayers to receive a bitmap along with a mailbox

cc/layers/texture_layer.cc

Index: cc/layers/texture_layer.cc
diff --git a/cc/layers/texture_layer.cc b/cc/layers/texture_layer.cc
index a6022dd2408faf496db47913187e4d4864f0bf70..6fb4c96b44215ca60b002112306edb35c6adf7c0 100644
--- a/cc/layers/texture_layer.cc
+++ b/cc/layers/texture_layer.cc
@@ -159,6 +159,10 @@ void TextureLayer::WillModifyTexture() {
   }
 }
 
+bool TextureLayer::MustUseSharedMemory() {
+  return !layer_tree_host()->UsingGLRenderer();

[piman, 2013/07/01 22:45:01]

note: layer_tree_host() is NULL when the layer is outside of the tree.
More generally we may construct layer trees before we know anything about the
system capabilities. I'm not sure UsingGLRenderer() is generally safe.

Maybe a better way would be to pass that info through PrepareTextureMailbox - I
believe that's the only place you need it currently. PrepareTextureMailbox is
called from TextureLayer::Update which is called from
LayerTreeHost::UpdateLayers, which has 2 advantages:
1- you know for sure you have a LTH
2- LTH::UpdateLayers is called from the BeginFrame flow, i.e. started from the
proxy/LTHI after proper initialization, when we have all the information (i.e.
similar to, say, the max texture size).

[jbauman, 2013/07/02 00:00:40]

Ok, modified PrepareTextureMailbox to do this instead.

+}
+
 void TextureLayer::SetNeedsDisplayRect(const gfx::RectF& dirty_rect) {
   Layer::SetNeedsDisplayRect(dirty_rect);