Use a mocked compositor for unit tests

ui/aura/test/test_suite.cc

Index: ui/aura/test/test_suite.cc
diff --git a/ui/aura/test/test_suite.cc b/ui/aura/test/test_suite.cc
index 5193de117af12656819e81a445f89c6387ea5388..ebe1a0d95adabb9e5bce911134fcfcc33248f579 100644
--- a/ui/aura/test/test_suite.cc
+++ b/ui/aura/test/test_suite.cc
@@ -9,6 +9,8 @@
 #include "build/build_config.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/base/ui_base_paths.h"
+#include "ui/gfx/compositor/compositor.h"
+#include "ui/gfx/compositor/test_compositor.h"
 #include "ui/gfx/gfx_paths.h"
 #include "ui/gfx/gl/gl_implementation.h"
 
@@ -30,6 +32,10 @@ void AuraTestSuite::Initialize() {
   // Force unittests to run using en-US so if we test against string
   // output, it'll pass regardless of the system language.
   ui::ResourceBundle::InitSharedInstance("en-US");
+
+  // Use a mock compositor that noops draws.
+  ui::Compositor::set_compositor_factory_for_testing(
+      ui::TestCompositor::Create);
 }
 
 void AuraTestSuite::Shutdown() {