Avoid depending on Promise.prototype.toString() in layout tests

Avoid depending on Promise.prototype.toString() in layout tests

This makes it possible to flip V8's behavior via the --harmony-tostring flag
without updating Blink tests.

R=arv@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196439

[adamk, 2015-06-03 20:43:35]

adamk@chromium.org changed reviewers:
+ arv@chromium.org

[arv (Not doing code reviews), 2015-06-03 20:44:55]

LGTM

[adamk, 2015-06-03 20:45:13]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-06-03 20:45:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1154523007/1

[commit-bot: I haz the power, 2015-06-03 23:01:18]

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

[commit-bot: I haz the power, 2015-06-03 23:01:19]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/65120)

[adamk, 2015-06-03 23:13:09]

Committed patchset #1 (id:1) manually as 196439 (presubmit successful).