Use a mocked compositor for unit tests

ui/gfx/compositor/test_compositor.h

Index: ui/gfx/compositor/test_compositor.h
diff --git a/ui/gfx/compositor/test_compositor.h b/ui/gfx/compositor/test_compositor.h
index b7d2cfa395e783ebdeaa8b6186ee762746282f33..f4f6300208108feb075275ca771d53e54f73954b 100644
--- a/ui/gfx/compositor/test_compositor.h
+++ b/ui/gfx/compositor/test_compositor.h
@@ -18,7 +18,7 @@ class TestCompositorDelegate;
 // Trivial Compositor implementation that creates Textures of type TestTexture.
 class TestCompositor : public ui::Compositor {
  public:
-  TestCompositor();
+  explicit TestCompositor(CompositorDelegate* owner);
   virtual ~TestCompositor();
 
   // ui::Compositor:
@@ -27,6 +27,9 @@ class TestCompositor : public ui::Compositor {
   virtual void OnNotifyEnd() OVERRIDE;
   virtual void Blur(const gfx::Rect& bounds) OVERRIDE;
 
+  // A simple factory that creates a test compositor with a given delegate
+  static ui::Compositor* Create(ui::CompositorDelegate* owner);
+
  protected:
   virtual void OnWidgetSizeChanged() OVERRIDE;