Use a mocked compositor for unit tests

ui/aura/desktop.cc

Index: ui/aura/desktop.cc
diff --git a/ui/aura/desktop.cc b/ui/aura/desktop.cc
index 243fffd9d155c7c36cf8ac4791235cf6cf7a0343..f29ea3d465b30e53d3d281e454d4dc6ebe866507 100644
--- a/ui/aura/desktop.cc
+++ b/ui/aura/desktop.cc
@@ -25,17 +25,14 @@ namespace aura {
 // static
 Desktop* Desktop::instance_ = NULL;
 
-// static
-ui::Compositor*(*Desktop::compositor_factory_)() = NULL;
-
 Desktop::Desktop()
     : delegate_(NULL),
       host_(aura::DesktopHost::Create(gfx::Rect(200, 200, 1280, 1024))),
       ALLOW_THIS_IN_INITIALIZER_LIST(schedule_paint_factory_(this)),
       active_window_(NULL),
       in_destructor_(false) {
-  if (compositor_factory_) {
-    compositor_ = (*Desktop::compositor_factory())();
+  if (ui::Compositor::compositor_factory_()) {
+    compositor_ = (*ui::Compositor::compositor_factory())(this);
   } else {
     compositor_ = ui::Compositor::Create(this, host_->GetAcceleratedWidget(),
                                          host_->GetSize());