This patch makes Promise.prototype.then use @@species as specified
in ES2015. The fix is hoped for by certain users, such as legacy
core.js versions which encounter an unhandled Promise reject (complete
with an ugly console message) when Promise subclassing is supported
in a mostly correct way, and we do error checking on Promise
constructors, but @@species is not supported.
BUG=chromium:575314,v8:4633
LOG=Y
R=adamk
Review URL: https://codereview.chromium.org/1577223002
Cr-Commit-Position: refs/heads/master@{#33225}
569145019d