Mac: Use appropriate mechanism to determine partial swap capability

ui/compositor/compositor.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/compositor/compositor.h"
 
 #include <algorithm>
 #include <deque>
 
 #include "base/bind.h"
@@ skipping 80 matching lines @@
   // When impl-side painting is enabled, this will ensure PictureLayers always
   // can have LCD text, to match the previous behaviour with ContentLayers,
   // where LCD-not-allowed notifications were ignored.
   settings.layers_always_allowed_lcd_text = true;
   settings.renderer_settings.refresh_rate =
       context_factory_->DoesCreateTestContexts() ? kTestRefreshRate
                                                  : kDefaultRefreshRate;
   settings.main_frame_before_activation_enabled = false;
   settings.throttle_frame_production =
       !command_line->HasSwitch(switches::kDisableGpuVsync);
-#if !defined(OS_MACOSX)
   settings.renderer_settings.partial_swap_enabled =
       !command_line->HasSwitch(cc::switches::kUIDisablePartialSwap);
-#endif
 #if defined(OS_CHROMEOS)
   settings.per_tile_painting_enabled = true;
 #endif
 #if defined(OS_WIN)
   settings.renderer_settings.finish_rendering_on_resize = true;
 #endif
 
   // These flags should be mirrored by renderer versions in content/renderer/.
   settings.initial_debug_state.show_debug_borders =
       command_line->HasSwitch(cc::switches::kUIShowCompositedLayerBorders);
@@ skipping 305 matching lines @@
                     observer_list_,
                     OnCompositingLockStateChanged(this));
 }
 
 void Compositor::CancelCompositorLock() {
   if (compositor_lock_)
     compositor_lock_->CancelLock();
 }
 
 }  // namespace ui