- Implements the Promise.race algorithm using CodeStubAssembler.
- Delete src/js/promise.js, which is no longer needed.
- Migrate Promise constructor from slow to fast object in bootstrapper
  (per v8:5902)

Increases size of snapshot_blob.bin on an x64.release build by 1.27kb.

BUG=v8:5343
R=gsathya@chromium.org, cbruni@chromium.org

Change-Id: I751e7389bd6ba410109640fcd7960b6021540f2f
Reviewed-on: https://chromium-review.googlesource.com/535041
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46489}