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 7824c128539dbbab4027ff30a7fec828f72ed9ba..69acf9e25568089f60a61c4cfc7f216a8945453b 100644
--- a/ui/aura/desktop.cc
+++ b/ui/aura/desktop.cc
@@ -44,9 +44,6 @@ static const int kDefaultHostWindowHeight = 1024;
 // static
 Desktop* Desktop::instance_ = NULL;
 
-// static
-ui::Compositor*(*Desktop::compositor_factory_)() = NULL;
-
 Desktop::Desktop()
     : Window(NULL),
       host_(aura::DesktopHost::Create(GetInitialHostWindowBounds())),
@@ -64,8 +61,8 @@ Desktop::Desktop()
   host_->SetDesktop(this);
   last_mouse_location_ = host_->QueryMouseLocation();
 
-  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());