Add decode() functionality to image elements.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index f02a36c93d9f84b42c6df08717ce4986ccd47332..43709fdf464db483f11a83cb3e52d38b632d7e71 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -122,6 +122,8 @@ class LayerTreeHostImplClient {
       bool resourceless_software_draw) = 0;
 
   virtual void NeedsImplSideInvalidation() = 0;
+  // Called when a requested image decode completes.
+  virtual void NotifyImageDecodeRequestFinished() = 0;
 
  protected:
   virtual ~LayerTreeHostImplClient() {}