Move NSWindow.allowsAutomaticWindowTabbing forward declaration to the right place.

When I added it in this CL:
https://codereview.chromium.org/2325313002/

…it worked for 10.11 (coincidentally) but not for the 10.12 SDK.

BUG=642155

Review-Url: https://codereview.chromium.org/2339563004
Cr-Commit-Position: refs/heads/master@{#418466}

base/mac/sdk_forward_declarations.h

@@ -38,10 +38,6 @@
 @protocol CALayerDelegate
 @end
 
-@interface NSWindow (SierraSDK)
-@property(class) BOOL allowsAutomaticWindowTabbing;
-@end
-
 #endif  // MAC_OS_X_VERSION_10_12
 
 #if !defined(MAC_OS_X_VERSION_10_11) || \
@@ -244,6 +240,17 @@ BASE_EXPORT extern NSString* const NSAppearanceNameVibrantLight;
 
 #endif  // MAC_OS_X_VERSION_10_10
 
+// Once Chrome no longer supports OSX 10.11, everything within this
+// preprocessor block can be removed.
+#if !defined(MAC_OS_X_VERSION_10_12) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
+
+@interface NSWindow (SierraSDK)
+@property(class) BOOL allowsAutomaticWindowTabbing;
+@end
+
+#endif  // MAC_OS_X_VERSION_10_12
+
 // ----------------------------------------------------------------------------
 // The symbol for kCWSSIDDidChangeNotification is available in the
 // CoreWLAN.framework for OSX versions 10.6 through 10.10. The symbol is not