Add a fast-path to Promise#finally, which skips the "then" lookup of the
Promise#then lookup chain is intact, similar to what we already do for
Promise#catch.

Drive-by-fix: Also use the @@species protector to speed up the lookup
of the SpeciesConstructor in Promise#finally.

Bug: v8:7253
Change-Id: If77e779a0188904effc4528beffc8f0bdd7c2efe
Reviewed-on: https://chromium-review.googlesource.com/902283Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51116}