Prepare ui/ module for compilation with OS X 10.7 deployment target.

ui/gfx/screen_mac.mm

 // 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/gfx/screen.h"
 
 #import <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 #include <stdint.h>
 
@@ skipping 54 matching lines @@
   // Convert work area's coordinate systems.
   if ([screen isEqual:primary]) {
     gfx::Rect work_area = gfx::Rect(NSRectToCGRect(visible_frame));
     work_area.set_y(frame.size.height - visible_frame.origin.y -
                     visible_frame.size.height);
     display.set_work_area(work_area);
   } else {
     display.set_bounds(ConvertCoordinateSystem(frame));
     display.set_work_area(ConvertCoordinateSystem(visible_frame));
   }
-  CGFloat scale;
-  if ([screen respondsToSelector:@selector(backingScaleFactor)])
-    scale = [screen backingScaleFactor];
-  else
-    scale = [screen userSpaceScaleFactor];
+  CGFloat scale = [screen backingScaleFactor];
 
   if (gfx::Display::HasForceDeviceScaleFactor())
     scale = gfx::Display::GetForcedDeviceScaleFactor();
 
   display.set_device_scale_factor(scale);
   // CGDisplayRotation returns a double. Display::SetRotationAsDegree will
   // handle the unexpected situations were the angle is not a multiple of 90.
   display.SetRotationAsDegree(static_cast<int>(CGDisplayRotation(display_id)));
   return display;
 }
@@ skipping 185 matching lines @@
 
 namespace gfx {
 
 #if !defined(USE_AURA)
 Screen* CreateNativeScreen() {
   return new ScreenMac;
 }
 #endif
 
 }