Do not use a Blink feature flag for Manifest.

Source/core/html/HTMLLinkElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Rob Buis (rwlbuis@gmail.com)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 192 matching lines @@
 {
     bool visible = inDocument() && !m_isInShadowTree;
     if (!visible) {
         ASSERT(!linkStyle() || !linkStyle()->hasSheet());
         return 0;
     }
 
     if (!m_link) {
         if (m_relAttribute.isImport()) {
             m_link = LinkImport::create(this);
-        } else if (m_relAttribute.isManifest() && RuntimeEnabledFeatures::manifestEnabled()) {
+        } else if (m_relAttribute.isManifest()) {
             m_link = LinkManifest::create(this);
         } else {
             OwnPtrWillBeRawPtr<LinkStyle> link = LinkStyle::create(this);
             if (fastHasAttribute(disabledAttr) || m_relAttribute.isTransitionExitingStylesheet())
                 link->setDisabledState(true);
             m_link = link.release();
         }
     }
 
     return m_link.get();
@@ skipping 514 matching lines @@
         removePendingSheet();
 }
 
 void LinkStyle::trace(Visitor* visitor)
 {
     visitor->trace(m_sheet);
     LinkResource::trace(visitor);
 }
 
 } // namespace blink