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

ui/base/layout_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/base/layout.h"
 
 #include <Cocoa/Cocoa.h>
 
 #include "base/mac/sdk_forward_declarations.h"
 
 namespace {
 
 float GetScaleFactorScaleForNativeView(gfx::NativeView view) {
   if (NSWindow* window = [view window]) {
-    if ([window respondsToSelector:@selector(backingScaleFactor)])
-      return [window backingScaleFactor];
-    return [window userSpaceScaleFactor];
+    return [window backingScaleFactor];
   }
 
   NSArray* screens = [NSScreen screens];
   if (![screens count])
     return 1.0f;
 
   NSScreen* screen = [screens objectAtIndex:0];
-  if ([screen respondsToSelector:@selector(backingScaleFactor)])
-    return [screen backingScaleFactor];
-  return [screen userSpaceScaleFactor];
+  return [screen backingScaleFactor];
 }
 
 }  // namespace
 
 namespace ui {
 
 float GetScaleFactorForNativeView(gfx::NativeView view) {
   return GetScaleFactorScaleForNativeView(view);
 }
 
 }  // namespace ui