[promise] separate PerformPromiseThen from PromiseThen

[promise] separate PerformPromiseThen from PromiseThen

The `PerformPromiseThen` spec-internal operation is used by the async functions
proposal, in order to ensure that AwaitExpressions are not observable via
usual mechanisms/hooks, such as Symbol.species.

This patch has a small optimization over the specification's language to short-
circuit new Promise creation when unnecessary, similar to the logic in
Promise.resolve.

BUG=v8:5253
R=caitp@igalia.com, adamk@chromium.org, gsathya@chromium.org, yangguo@chromium.org

[Dan Ehrenberg, 2016-08-04 20:37:41]

The CQ bit was checked by littledan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-08-04 20:37:44]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-04 21:03:52]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-08-04 21:03:52]

Dry run: Try jobs failed on following builders:
  v8_win64_rel_ng on master.tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_win64_rel_ng/builds/11888)
  v8_win64_rel_ng_triggered on master.tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_win64_rel_ng_triggered/b...)