TryCatch only clears the pending exception if it has been propagated
through OptionalRescheduleException. This is another tentative fix for
https://crbug.com/754422.

Bug: chromium:754422
Change-Id: Ifbbeed8ef44131a0a010ac6bde3adbbf9fb4c4af
Reviewed-on: https://chromium-review.googlesource.com/637305Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47634}