Don't layout twice in ui::Compositor

In the SingleThreadProxy+SchedulerClient patch,
LayerTreeHostClient::Layout is now called via DoCommit which is called
via Composite(Immediately).  In no cases should the embedder call
Layout() manually.

R=danakj@chromium.org,piman@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/545343003

Cr-Commit-Position: refs/heads/master@{#293632}

ui/compositor/compositor.cc

@@ -231,7 +231,6 @@ void Compositor::Draw() {
                                    base::TimeTicks(),
                                    cc::BeginFrameArgs::DefaultInterval());
     BeginMainFrame(args);
-    Layout();
     host_->Composite(args.frame_time);
   }
   if (swap_state_ == SWAP_NONE)