Don't attempt to revalidate a Resource with a 304 response.

third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.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
@@ skipping 691 matching lines @@
         return Reload;
 
     // If resource was populated from a SubstituteData load or data: url, use it.
     if (isStaticData)
         return Use;
 
     if (!existingResource->canReuse(request))
         return Reload;
 
     // Certain requests (e.g., XHRs) might have manually set headers that require revalidation.
-    // FIXME: In theory, this should be a Revalidate case. In practice, the MemoryCache revalidation path assumes a whole bunch
+    // In theory, this should be a Revalidate case. In practice, the MemoryCache revalidation path assumes a whole bunch
     // of things about how revalidation works that manual headers violate, so punt to Reload instead.
-    if (request.isConditional())
+    //
+    // Similarly, a request with manually added revalidation headers can lead
+    // to a 304 response for a request that wasn't flagged as a revalidation
+    // attempt. Normally, successful revalidation will maintain the original
+    // response's status code, but for a manual revalidation the response code
+    // remains 304. In this case, the Resource likely has insufficient context
+    // to provide a useful cache hit or revalidation.
+    // See http://crbug.com/643659
+    if (request.isConditional() || existingResource->response().httpStatusCode() == 304)
         return Reload;
 
     // Don't try to reuse an in-progress async request for a new sync request.
     if (fetchRequest.options().synchronousPolicy == RequestSynchronously && existingResource->isLoading())
         return Reload;
 
     // Don't reload resources while pasting.
     if (m_allowStaleResources)
         return Use;
 
@@ skipping 635 matching lines @@
     visitor->trace(m_context);
     visitor->trace(m_archive);
     visitor->trace(m_loaders);
     visitor->trace(m_nonBlockingLoaders);
     visitor->trace(m_documentResources);
     visitor->trace(m_preloads);
     visitor->trace(m_resourceTimingInfoMap);
 }
 
 } // namespace blink