Make multipart image documents work again.

Source/core/fetch/Resource.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple 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
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 152 matching lines @@
 
     virtual bool isImage() const { return false; }
     bool shouldBlockLoadEvent() const
     {
         return type() != LinkPrefetch
             && type() != LinkSubresource
             && type() != Media
             && type() != Raw
             && type() != TextTrack;
     }
+    bool canBeMultipart() const { return type() == Image || type() == MainResource; }

[Mike West, 2015/05/28 05:51:54]

This seems unclear; we don't support multipart for _any_ MainResource, do we?
What happens if we try to load a non-image multipart document?

[Nate Chapin, 2015/05/28 17:44:24]

We fail. I was hoping to convey some level of ambiguity with "can" in the
function name. Maybe leave this inlined with a comment in
ResourceLoader::didReceiveResponse() is clearer?

 
     // Computes the status of an object after loading.
     // Updates the expire date on the cache entry file
     void setLoadFinishTime(double finishTime) { m_loadFinishTime = finishTime; }
     void finish();
 
     // FIXME: Remove the stringless variant once all the callsites' error messages are updated.
     bool passesAccessControlCheck(SecurityOrigin*) const;
     bool passesAccessControlCheck(SecurityOrigin*, String& errorDescription) const;
 
@@ skipping 238 matching lines @@
 const char* ResourceTypeName(Resource::Type);
 #endif
 
 #define DEFINE_RESOURCE_TYPE_CASTS(typeName) \
     DEFINE_TYPE_CASTS(typeName##Resource, Resource, resource, resource->type() == Resource::typeName, resource.type() == Resource::typeName); \
     inline typeName##Resource* to##typeName##Resource(const ResourcePtr<Resource>& ptr) { return to##typeName##Resource(ptr.get()); }
 
 }
 
 #endif