Aura: Make compositor_unittests work with --ui-enable-threaded-compositing

ui/compositor/test/test_compositor_host_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/compositor/test/test_compositor_host.h"
 
 #import <AppKit/NSApplication.h>
 #import <AppKit/NSOpenGL.h>
 #import <AppKit/NSView.h>
 #import <AppKit/NSWindow.h>
@@ skipping 120 matching lines @@
   [view setCompositor:compositor_.get()];
   [window_ setContentView:view];
   [window_ orderFront:nil];
 }
 
 ui::Compositor* TestCompositorHostMac::GetCompositor() {
   return compositor_.get();
 }
 
 void TestCompositorHostMac::ScheduleDraw() {
+  DCHECK(!ui::Compositor::WasInitializedWithThread());
   if (!compositor_.get())
     return;
 
   // Force display now.
   [window_ display];
 }
 
 // static
 TestCompositorHost* TestCompositorHost::Create(const gfx::Rect& bounds) {
   return new TestCompositorHostMac(bounds);
 }
 
 }  // namespace ui