The IsPromise brand check is now replaced with an IsObject check.

The spec was changed here:
https://github.com/tc39/proposal-promise-finally/commit/a1628886f85a897df5cd967ea36f025e8f89cb7a

Bug: v8:7095
Change-Id: I5668083c888f9efcdfc1491c919c810c75d73ac7
Reviewed-on: https://chromium-review.googlesource.com/826606Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50108}