aura: Clean up compositor initialization/destruction.

ash/test/test_shell_delegate.cc

Index: ash/test/test_shell_delegate.cc
diff --git a/ash/test/test_shell_delegate.cc b/ash/test/test_shell_delegate.cc
index 532876ec851a7f44b5202adac74ca45a65f1aea7..a21eddcd03c10862576a59ca287a86db3d9e21a9 100644
--- a/ash/test/test_shell_delegate.cc
+++ b/ash/test/test_shell_delegate.cc
@@ -17,6 +17,7 @@
 #include "base/logging.h"
 #include "content/public/test/test_browser_context.h"
 #include "ui/aura/window.h"
+#include "ui/compositor/compositor.h"
 
 namespace ash {
 namespace test {
@@ -213,6 +214,10 @@ double TestShellDelegate::GetSavedScreenMagnifierScale() {
 }
 
 RootWindowHostFactory* TestShellDelegate::CreateRootWindowHostFactory() {
+  // The ContextFactory must exist before any Compositors are created.
+  bool allow_test_contexts = true;
+  ui::Compositor::InitializeContextFactoryForTests(allow_test_contexts);
+
   return RootWindowHostFactory::Create();
 }