Add a CHECK to investigate crash in OAuth2ManifestHandler::Parse

Add a CHECK to investigate crash in OAuth2ManifestHandler::Parse

I don't see anything wrong with the code, so adding a CHECK to investigate
further. If there are no crashes on this CHECK it can be removed in M42.

BUG=445683
TEST=extensions_unittests

Committed: https://crrev.com/ec299aa0ff64e7b4647d376c93d6927ecc85d070
Cr-Commit-Position: refs/heads/master@{#310107}

[James Cook, 2015-01-05 22:21:48]

jamescook@chromium.org changed reviewers:
+ courage@chromium.org

[James Cook, 2015-01-05 22:21:48]

courage, PTAL

[Michael Courage, 2015-01-05 23:29:11]

lgtm

[James Cook, 2015-01-06 00:09:25]

jamescook@chromium.org changed reviewers:
+ rockot@chromium.org

[James Cook, 2015-01-06 00:09:25]

rockot, can I have OWNERS?

[Ken Rockot(use gerrit already), 2015-01-06 04:05:06]

On 2015/01/06 00:09:25, James Cook wrote:
> rockot, can I have OWNERS?

lgtm

[James Cook, 2015-01-06 16:49:39]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2015-01-06 16:50:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/832333008/1

[commit-bot: I haz the power, 2015-01-06 17:55:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-06 17:56:08]

Patchset 1 (id:??) landed as
https://crrev.com/ec299aa0ff64e7b4647d376c93d6927ecc85d070
Cr-Commit-Position: refs/heads/master@{#310107}