Use a mocked compositor for unit tests

ui/gfx/compositor/compositor.h

Index: ui/gfx/compositor/compositor.h
diff --git a/ui/gfx/compositor/compositor.h b/ui/gfx/compositor/compositor.h
index 574fed88f87ce3475fa21732ef5c79a91b478556..4ec89da00283d7e34150bdd34601c6ba8b35b9ea 100644
--- a/ui/gfx/compositor/compositor.h
+++ b/ui/gfx/compositor/compositor.h
@@ -137,6 +137,16 @@ class COMPOSITOR_EXPORT Compositor : public base::RefCounted<Compositor> {
   void RemoveObserver(CompositorObserver* observer);
   bool HasObserver(CompositorObserver* observer);
 
+  static void set_compositor_factory_for_testing(
+      ui::Compositor*(*factory)(ui::CompositorDelegate* owner)) {
+    compositor_factory_ = factory;
+  }
+
+  static ui::Compositor* (*compositor_factory())(
+      ui::CompositorDelegate* owner) {
+    return compositor_factory_;
+  }
+
  protected:
   Compositor(CompositorDelegate* delegate, const gfx::Size& size);
   virtual ~Compositor();
@@ -169,6 +179,12 @@ class COMPOSITOR_EXPORT Compositor : public base::RefCounted<Compositor> {
 
   ObserverList<CompositorObserver> observer_list_;
 
+  // Factory used to create Compositors. Settable by tests.
+  // The delegate can be NULL if you don't wish to catch the ScheduleDraw()
+  // calls to it.
+  static ui::Compositor*(*compositor_factory_)(
+      ui::CompositorDelegate* delegate);
+
   friend class base::RefCounted<Compositor>;
 };