Mac: Use appropriate mechanism to determine partial swap capability

BUG=474299

Review URL: https://codereview.chromium.org/1059483004

Cr-Commit-Position: refs/heads/master@{#323992}

content/common/gpu/image_transport_surface_fbo_mac.mm

@@ -209,7 +209,7 @@ bool ImageTransportSurfaceFBO::PostSubBuffer(
 }
 
 bool ImageTransportSurfaceFBO::SupportsPostSubBuffer() {
-  return true;
+  return false;
 }
 
 gfx::Size ImageTransportSurfaceFBO::GetSize() {

ui/compositor/compositor.cc

@@ -98,10 +98,8 @@ Compositor::Compositor(gfx::AcceleratedWidget widget,
   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