This one allows us to support custom promises implementation.
With awaitPromise flag Runtime.evaluate awaits
Promise.resolve(<expression result>).
This also allows to await for any non-Promise value, similar to await
expression, which is more convenient for most protocol users.

R=dgozman@chromium.org

Bug: chromium:755104
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iee798b33b6fb7de7d393372e164c0481d1bbf7eb
Reviewed-on: https://chromium-review.googlesource.com/614308
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47354}