Move the CSS Device Adaptation @viewport rule support behind a runtime flag

Source/core/css/CSSRule.h

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2006, 2007, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Andreas Kling (kling@webkit.org)
  *
  * 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.
@@ skipping 33 matching lines @@
         IMPORT_RULE,
         MEDIA_RULE,
         FONT_FACE_RULE,
         PAGE_RULE,
         // 7 was VARIABLES_RULE; we now match other browsers with 7 as
         // KEYFRAMES_RULE:
         // <https://bugs.webkit.org/show_bug.cgi?id=71293>.
         WEBKIT_KEYFRAMES_RULE,
         WEBKIT_KEYFRAME_RULE,
         SUPPORTS_RULE = 12,
-#if ENABLE(CSS_DEVICE_ADAPTATION)
         WEBKIT_VIEWPORT_RULE = 15,
-#endif
         WEBKIT_REGION_RULE = 16,
         WEBKIT_FILTER_RULE = 17,
         HOST_RULE = 1001,
     };
 
     virtual Type type() const = 0;
     virtual String cssText() const = 0;
     virtual void reattach(StyleRuleBase*) = 0;
     virtual void reportMemoryUsage(MemoryObjectInfo*) const;
 
@@ skipping 40 matching lines @@
 
     union {
         CSSRule* m_parentRule;
         CSSStyleSheet* m_parentStyleSheet;
     };
 };
 
 } // namespace WebCore
 
 #endif // CSSRule_h