Don't trigger accelerated compositing for 3D CSS when using SwiftShader

content/browser/tab_contents/tab_contents.cc

Index: content/browser/tab_contents/tab_contents.cc
diff --git a/content/browser/tab_contents/tab_contents.cc b/content/browser/tab_contents/tab_contents.cc
index d51951d7b68bdc5a92fbcd08d26d31978154bf83..45a6ca380565a6e914f2801abc32b3ac61eb6cf7 100644
--- a/content/browser/tab_contents/tab_contents.cc
+++ b/content/browser/tab_contents/tab_contents.cc
@@ -431,10 +431,11 @@ WebPreferences TabContents::GetWebkitPrefs(RenderViewHost* rvh,
       prefs.gl_multisampling_enabled = false;
 
     // Accelerated video and animation are slower than regular when using a
-    // software 3d rasterizer.
+    // software 3d rasterizer. 3D CSS may also be too slow to be worthwhile.
     if (gpu_data_manager->ShouldUseSoftwareRendering()) {
       prefs.accelerated_video_enabled = false;
       prefs.accelerated_animation_enabled = false;
+      prefs.accelerated_layers_enabled = false;
     }
   }