Port cdae8654

Original commit message:

    A corresponding flag was added as well to help us find out what breaks when we
    do not clear pending exceptions on each JS entry.

R=jgruber@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=5259
LOG=N

Review-Url: https://codereview.chromium.org/2208073003
Cr-Commit-Position: refs/heads/master@{#38312}