mac: Suppress partial availability warnings in theme helper.

Moved the definition of a 10.7+ NSString constant into
mac/sdk_forward_declarations.h, and ensured that the NSString is explicitly
defined regardless of the OSX SDK version being compiled against.

BUG=471823

Review URL: https://codereview.chromium.org/1133523005

Cr-Commit-Position: refs/heads/master@{#329459}

base/mac/sdk_forward_declarations.h

@@ -244,6 +244,8 @@ BASE_EXPORT extern NSString* const NSWindowDidExitFullScreenNotification;
 BASE_EXPORT extern NSString* const
     NSWindowDidChangeBackingPropertiesNotification;
 BASE_EXPORT extern NSString* const CBAdvertisementDataServiceDataKey;
+BASE_EXPORT extern NSString* const
+    NSPreferredScrollerStyleDidChangeNotification;
 #endif  // MAC_OS_X_VERSION_10_7
 
 #if !defined(MAC_OS_X_VERSION_10_9) || \

base/mac/sdk_forward_declarations.mm

@@ -22,6 +22,9 @@ NSString* const NSWindowDidChangeBackingPropertiesNotification =
     @"NSWindowDidChangeBackingPropertiesNotification";
 
 NSString* const CBAdvertisementDataServiceDataKey = @"kCBAdvDataServiceData";
+
+NSString* const NSPreferredScrollerStyleDidChangeNotification =
+    @"NSPreferredScrollerStyleDidChangeNotification";
 #endif  // MAC_OS_X_VERSION_10_7
 
 #if !defined(MAC_OS_X_VERSION_10_9) || \

content/browser/theme_helper_mac.mm

@@ -15,15 +15,6 @@
 #include "content/public/browser/render_process_host.h"
 #include "content/public/common/content_switches.h"
 
-// Declare notification names from the 10.7 SDK.
-#if !defined(MAC_OS_X_VERSION_10_7) || \
-    MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
-
-NSString* NSPreferredScrollerStyleDidChangeNotification =
-    @"NSPreferredScrollerStyleDidChangeNotification";
-
-#endif
-
 @interface ScrollbarPrefsObserver : NSObject
 
 + (void)registerAsObserver;