Makes copying layers clone external textures.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index c150fb3733c6680161acfc9ec77c31f982772608..f9fcbc9208246dbb3eb655747676951ea4f70368 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -220,6 +220,7 @@ class COMPOSITOR_EXPORT Layer
   // Assigns a new external texture.  |texture| can be NULL to disable external
   // updates.
   void SetExternalTexture(ui::Texture* texture);
+  ui::Texture* external_texture() { return texture_.get(); }
 
   // Sets the layer's fill color.  May only be called for LAYER_SOLID_COLOR.
   void SetColor(SkColor color);