Remove CSS_REGIONS compile flag from Blink.

Source/core/css/CSSRule.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 22 matching lines @@
     const unsigned short IMPORT_RULE = 3;
     const unsigned short MEDIA_RULE = 4;
     const unsigned short FONT_FACE_RULE = 5;
     const unsigned short PAGE_RULE = 6;
     const unsigned short WEBKIT_KEYFRAMES_RULE = 7;
     const unsigned short WEBKIT_KEYFRAME_RULE = 8;
     const unsigned short SUPPORTS_RULE = 12;
 #if defined(ENABLE_CSS_DEVICE_ADAPTATION) && ENABLE_CSS_DEVICE_ADAPTATION
     const unsigned short WEBKIT_VIEWPORT_RULE = 15;
 #endif
-#if defined(ENABLE_CSS_REGIONS) && ENABLE_CSS_REGIONS
-    const unsigned short WEBKIT_REGION_RULE = 16;
-#endif
+    [EnabledAtRuntime=cssRegions] const unsigned short WEBKIT_REGION_RULE = 16;
     const unsigned short WEBKIT_FILTER_RULE = 17;
     const unsigned short HOST_RULE = 1001;
 
     readonly attribute unsigned short   type;
 
              [TreatReturnedNullStringAs=Null, TreatNullAs=NullString, SetterRaisesException] attribute DOMString        cssText;
 
     readonly attribute CSSStyleSheet    parentStyleSheet;
     readonly attribute CSSRule          parentRule;
 
 };