Revert of [runtime] temporarily transform IsContext check from DHECK to CHECK (patchset #2 id:20001 of https://codereview.chromium.org/2164633002/ )

Reason for revert:
There have been no more occurrences of this on dev / beta so we can convert the CHECK back to DCHECK.

Original issue's description:
> [runtime] temporarily transform IsContext check from DCHECK to CHECK
>
> We are enabling this trial on canary to see if we can flush out some missing
> context restores.
>
> BUG=
>
> Committed: https://crrev.com/ec94ad400dc257af396efa3b1899bc3168347d82
> Cr-Commit-Position: refs/heads/master@{#37875}

TBR=jkummerow@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.chromium.org/2303543003
Cr-Commit-Position: refs/heads/master@{#39292}