The pattern of creating a hidden catch scope occurs several times, so
define a helper for it.

BUG=

Change-Id: I5831378341d86d41bfb015bd3e687183cc79ac20
Reviewed-on: https://chromium-review.googlesource.com/448559
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43574}