Remove already obsolete CREDENTIAL frame from SPDY code.

Remove already obsolete CREDENTIAL frame from SPDY code.

Support for CREDENTIAL frames was removed long ago (when SPDY/3 support was
removed).  This CL cleans up remaining unused data members, methods, enum
values, constants, tests, and comments.

Functionality change: incoming CREDENTIAL frames used to be ignored, now they
cause an error.

BUG=311852
Committed: https://crrev.com/99fbe5b572d435466e380334a947401ed15d11d8
Cr-Commit-Position: refs/heads/master@{#360974}

[Bence, 2015-11-20 15:35:21]

bnc@chromium.org changed reviewers:
+ rch@chromium.org

[Bence, 2015-11-20 15:35:22]

Ryan: PTAL.  Thanks.

[Ryan Hamilton, 2015-11-20 23:30:01]

lgtm

[Bence, 2015-11-21 00:48:31]

The CQ bit was checked by bnc@chromium.org

[commit-bot: I haz the power, 2015-11-21 00:49:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1461273003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1461273003/1

[commit-bot: I haz the power, 2015-11-21 01:07:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-21 01:09:14]

Patchset 1 (id:??) landed as
https://crrev.com/99fbe5b572d435466e380334a947401ed15d11d8
Cr-Commit-Position: refs/heads/master@{#360974}