The idea here is that in case the `thenable` is a JSPromise and `then`
is the initial `Promise.prototype.then` method, and the @@species lookup
chain is intact, we can skip creating the temporary promise and the
closures (with the shared context), and instead directly call into our
PerformPromiseThen. This is sound since - given above mentioned
conditions - our short-cut

  PerformPromiseThen(thenable, undefined, undefined, promise_to_resolve)

is not observably different from the actual

  resolve, reject = CreateResolvingFunctions(promise_to_resolve)
  result_capability = NewPromiseCapability(%Promise%)
  PerformPromiseThen(thenable, resolve, reject, result_capability)

except through PromiseHooks (and potentially via the async stack
traces). So we disable the fast-path if either promise hooks are enabled
or the debugger is active for now.

This improves the performance on the wikipedia benchmark by 20-25% and
the bluebird-doxbee benchmark by around 20%.

Bug: v8:7253
Change-Id: I23c92ad365c2b71d65057573f2d8febe2afe00b0
Reviewed-on: https://chromium-review.googlesource.com/911800
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51261}