mac: Move more 10.7 api stuff into sdk_forward_declarations.h

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>
 
-#if !defined(MAC_OS_X_VERSION_10_7) || \
-    MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
-
-@interface NSScreen (LionAPI)
-- (CGFloat)backingScaleFactor;
-@end
-
-@interface NSWindow (LionAPI)
-- (CGFloat)backingScaleFactor;
-@end
-
-#endif  // 10.7
+#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];
   }
 
   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];
 }
 
 }  // namespace
 
 namespace ui {
 
 ScaleFactor GetScaleFactorForNativeView(gfx::NativeView view) {
   return GetScaleFactorFromScale(GetScaleFactorScaleForNativeView(view));
 }
 
 }  // namespace ui