Enable by default threaded compositing on windows and FCM on mac

chrome/browser/task_manager/task_manager_browsertest.cc

Index: chrome/browser/task_manager/task_manager_browsertest.cc
diff --git a/chrome/browser/task_manager/task_manager_browsertest.cc b/chrome/browser/task_manager/task_manager_browsertest.cc
index 68f26001fdf71579a0b12ed525c27107cc3bd8e2..bf9ca8cd7b760fffc001c72bc76a0f98b5f70b7b 100644
--- a/chrome/browser/task_manager/task_manager_browsertest.cc
+++ b/chrome/browser/task_manager/task_manager_browsertest.cc
@@ -67,10 +67,12 @@ class TaskManagerBrowserTest : public ExtensionBrowserTest {
   virtual void SetUpCommandLine(CommandLine* command_line) OVERRIDE {
     ExtensionBrowserTest::SetUpCommandLine(command_line);
 
-    // Do not prelaunch the GPU process for these tests because it will show
-    // up in task manager but whether it appears before or after the new tab
-    // renderer process is not well defined.
+    // Do not prelaunch the GPU process and disable accelerated compositing
+    // for these tests as the GPU process will show up in task manager but
+    // whether it appears before or after the new tab renderer process is not
+    // well defined.
     command_line->AppendSwitch(switches::kDisableGpuProcessPrelaunch);
+    command_line->AppendSwitch(switches::kDisableAcceleratedCompositing);
   }
 };