This implements the ideas outlined in the section "Microtask queue"
of the exploration document "Promise and async/await performance" (at
https://goo.gl/WHRar2), except that the microtask queue stays a linear
FixedArray for now, to avoid running into trouble with the parallel
scavenger. This way we can already save a significant amount of
allocations, thereby reducing the GC frequency quite a bit.

All items on the microtask queue are now proper structs that subclass
Microtask, i.e. we also wrap JSFunction and MicrotaskCallback jobs
into structs. We also consistently remember the context for every
microtask (except for MicrotaskCallback where we don't have a
context), and execute it later in exactly that context (as required
by the spec anyways for the Promise related jobs). Particularly
interesting is the PromiseReactionJobTask and its subclasses, since
they are designed to have the same size as the PromiseReaction. When
we resolve a JSPromise we just take the existing PromiseReaction
instances and morph them into PromiseFulfillReactionJobTask or
PromiseRejectReactionJobTask (depending whether you "Fulfill" or
"Reject"). That way the JSPromise class is now only 6 words instead
of 10 words.

Also the PromiseReaction and the reaction tasks can either carry a
JSPromise (for the fast native case) or a PromiseCapability (for the
generic case), which means we don't always pay the overhead of having
to also remember the "deferred resolve" and "deferred reject" handlers
that are only relevant for the generic case anyways.

It also fixes a spec violation where we called "then" before we actually
enqueued the PromiseResolveThenableJob, which is observably wrong.
Calling it later has the advantage that it should be fairly
straight-forward now to completely avoid it for native Promise
instances.

This seems to save around 10-20% on the various Promise benchmarks and
micro-benchmarks. We expect to gain even more as we're now able to
inline various operations into TurboFan optimized code easily.

Bug: v8:7253
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I893d24ca5bb046974b4f5826a8f6dd22f1210b6a
Reviewed-on: https://chromium-review.googlesource.com/892819
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50980}