Add decode() functionality to image elements.

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index 3e9487ea358c701f9261f99567527d2442c46282..9a41cd0987c8b7e3ddac14ed60067908604f7ad9 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -97,6 +97,7 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
   void DidCompletePageScaleAnimationOnImplThread() override;
   void OnDrawForCompositorFrameSink(bool resourceless_software_draw) override;
   void NeedsImplSideInvalidation() override;
+  void NotifyImageDecodeRequestFinished() override;
 
   void RequestNewCompositorFrameSink();
 
@@ -120,6 +121,7 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
 
   bool ShouldComposite() const;
   void ScheduleRequestNewCompositorFrameSink();
+  void IssueImageDecodeFinishedCallbacks();
 
   // Accessed on main thread only.
   LayerTreeHost* layer_tree_host_;