Clearing the pending exception is not enough - if we want to swallow an
exception while currently on top of an external handler (e.g. TryCatch),
we also need to clear external_caught_exception.

BUG=chromium:719380

Review-Url: https://codereview.chromium.org/2870423002
Cr-Commit-Position: refs/heads/master@{#45247}