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 0b72dc47333e5f2da6a473840d3704e5d9fea5b9..8c05b60c6a20458e968e2e2b39b08f9a8f2aa360 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -123,6 +123,8 @@ 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() {}