Mirror API for IsAsyncFunction

Mirror API for IsAsyncFunction

DevTools wants to be able to detect async functions in order to print
their synopsis better in stack traces and tooltips. This patch provides a
simple method to do the check.

BUG=v8:4483

[Dan Ehrenberg, 2016-09-23 05:18:51]

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

[commit-bot: I haz the power, 2016-09-23 05:18:59]

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

[Dan Ehrenberg, 2016-09-23 05:21:50]

littledan@chromium.org changed reviewers:
+ jochen@chromium.org

[Dan Ehrenberg, 2016-09-23 05:21:51]

This patch provides one possible API in V8 to allow DevTools to check whether a
function is an async function. The advantage of this API is that it doesn't
pollute the already-large v8.h, and it doesn't tempt ordinary users to look into
this implementation detail of functions (async functions should be treated just
like any other function that returns a Promise).

jochen, if you have time, a review would be great, as the V8 API owner. An
alternative API is at https://codereview.chromium.org/2365833002 .

[commit-bot: I haz the power, 2016-09-23 07:19:39]

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

[commit-bot: I haz the power, 2016-09-23 07:19:40]

Dry run: Try jobs failed on following builders:
  v8_mac_rel_ng on master.tryserver.v8 (JOB_TIMED_OUT, no build URL)
  v8_win64_rel_ng on master.tryserver.v8 (JOB_TIMED_OUT, no build URL)
  v8_win_compile_dbg on master.tryserver.v8 (JOB_TIMED_OUT, no build URL)
  v8_win_nosnap_shared_rel_ng on master.tryserver.v8 (JOB_TIMED_OUT, no build
URL)
  v8_win_rel_ng on master.tryserver.v8 (JOB_TIMED_OUT, no build URL)