Revert of Avoid Context::Enter and Context::Exit if we're reentering the active and last entered context (patchset #2 id:20001 of https://codereview.chromium.org/2131483002/ )

Reason for revert:
crashes: crbug.com/626719

Original issue's description:
> Avoid Context::Enter and Context::Exit if we're reentering the active and last entered context
>
> A bit of browsing around indicates that the new fast-path is taken most of the time:
>    3496 Entering new
>  152295 Reentering same
>
> BUG=
>
> Committed: https://crrev.com/1829eb661ef1cf432fb551bcb193f521a219d490
> Cr-Commit-Position: refs/heads/master@{#37570}

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

Review-Url: https://codereview.chromium.org/2140563002
Cr-Commit-Position: refs/heads/master@{#37632}