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 6af5a94377e0d70328451795084ed7118f3718da..647249664e765faeca6457ac4a369d3df11d392d 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:
@@ -28,6 +28,9 @@ class TestCompositor : public ui::Compositor {
   virtual void Blur(const gfx::Rect& bounds) OVERRIDE;
   virtual void DrawTree() 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;