Port 5cef3ddd

Original Commit Message:

    Instead of requiring the pattern that a SuspendGenerator must be
    followed by a Return, make SuspendGenerator return directly. This can,
    in the future, simplify some of the reasoning around generator suspends.

R=leszeks@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ic967164b40433ed64df5996560490b13d6159a94
Reviewed-on: https://chromium-review.googlesource.com/879223Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#50774}