SPDY cleanup: remove credential slot.

Credential slot field has been removed from SYN_STREAM frame.

This lands server change 59587408 by mlavan.

Also update affected Chromium portions of QUIC & SPDY.

Review URL: https://codereview.chromium.org/141953004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247849 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247849