Disable threaded compositing, but leave force compositing

chrome/browser/gpu/chrome_gpu_util.cc

Index: chrome/browser/gpu/chrome_gpu_util.cc
diff --git a/chrome/browser/gpu/chrome_gpu_util.cc b/chrome/browser/gpu/chrome_gpu_util.cc
index 38339b9e6dc4c9e4f42e993d7defd4f79e218363..9fb72181f521b49bc43682d29e3963b6f3d886a6 100644
--- a/chrome/browser/gpu/chrome_gpu_util.cc
+++ b/chrome/browser/gpu/chrome_gpu_util.cc
@@ -156,9 +156,12 @@ void InitializeCompositingFieldTrial() {
   // http://crbug.com/133602 for mac
   // http://crbug.com/140866 for linux
 
-#if defined(OS_WIN) || defined(OS_MACOSX)
+#if defined(OS_WIN)
   // Enable threaded compositing on Windows.
   threaded_compositing_probability = kDivisor;
+#elif defined(OS_MACOSX)
+  // Enable force compositing mode on Mac.
+  force_compositing_mode_probability = kDivisor;
 #endif
 
   int force_compositing_group = trial->AppendGroup(