Add "StaleWhileRevalidateCacheControl" runtime flag.

public/web/WebRuntimeFeatures.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 135 matching lines @@
     BLINK_EXPORT static void enableSVG1DOM(bool);
 
     BLINK_EXPORT static void enableReducedReferrerGranularity(bool);
 
     BLINK_EXPORT static void enablePushMessaging(bool);
 
     BLINK_EXPORT static void enablePushMessagingData(bool);
 
     BLINK_EXPORT static void enablePushMessagingHasPermission(bool);
 
+    BLINK_EXPORT static void enableStaleWhileRevalidateCacheControl(bool);
+
     BLINK_EXPORT static void enableUnsafeES3APIs(bool);
 
     BLINK_EXPORT static void enableVRDevice(bool);
 private:
     WebRuntimeFeatures();
 };
 
 } // namespace blink
 
 #endif