Split the PromiseFinallyContextSlot enum into two separate enums
because the spec requires additional slot to store the Promise
constructor in the Promise.prototype.finally builtin. This will be
added in a follow on patch.

Inline the various context creation functions into their callsites since
they're only a single line and have only one callsite.

Bug: v8:5967
Change-Id: I2834c9c3d4940b8fbbdb7c162f42323d0fe0939f
Reviewed-on: https://chromium-review.googlesource.com/624543
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47525}