Use a mocked compositor for unit tests

content/test/content_test_suite.cc

Index: content/test/content_test_suite.cc
diff --git a/content/test/content_test_suite.cc b/content/test/content_test_suite.cc
index c962fb38a27308edcc6bc9aa225bee6b29996d09..1307aaa2d264b9fd907115e05b87f942d8b680ac 100644
--- a/content/test/content_test_suite.cc
+++ b/content/test/content_test_suite.cc
@@ -13,6 +13,7 @@
 #include "content/test/test_content_client.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/base/ui_base_paths.h"
+#include "ui/gfx/compositor/compositor_test_support.h"
 
 namespace {
 
@@ -65,6 +66,9 @@ void ContentTestSuite::Initialize() {
   content::RegisterPathProvider();
   ui::RegisterPathProvider();
 
+  // Mock out the compositor on platforms that use it.
+  ui::CompositorTestSupport::SetupMockCompositor();
+
   testing::TestEventListeners& listeners =
       testing::UnitTest::GetInstance()->listeners();
   listeners.Append(new TestContentClientInitializer);