Don’t smash globals used by the test helper itself.

Don’t smash globals used by the test helper itself.

Since https://codereview.chromium.org/366103005 the promise tests null out
various globals, to ensure the promise implementation doesn’t itself rely
on functions patchable by monkeys.

Unfortunately, doing so breaks test assertion failures which rely on
those globals.

This isn’t the ideal solution, but does improve the current state.

R=littledan@chromium.org,domenic@chromium.org
LOG=N
BUG=

Review URL: https://codereview.chromium.org/1370583002

Cr-Commit-Position: refs/heads/master@{#30922}