Unreviewed, rolling out r175406, r175413, and r175423.

Unreviewed, rolling out r175406, r175413, and r175423.
https://bugs.webkit.org/show_bug.cgi?id=138327

Multipart tests are still broken (Requested by ap on #webkit).

Reverted changesets:

"Eliminate ResourceBuffer and use SharedBuffer directly
instead"
https://bugs.webkit.org/show_bug.cgi?id=138174
http://trac.webkit.org/changeset/175406

"Unreviewed, iOS build fix since 175406."
http://trac.webkit.org/changeset/175413

"Fix assertion in CachedResource::addDataBuffer"
http://trac.webkit.org/changeset/175423

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@175491 268f45cc-cd09-0410-ab3c-d52691b4dbfc