Crashes are still happening despite tentative fixes, but unfortunately
without a local repro. This adds a couple of additional checks to help
flush out the root cause.

TBR=yangguo@chromium.org

Bug: chromium:754422
Change-Id: Ib3c8a2e0271fc724a4351ce6aec8298cf520a20a
Reviewed-on: https://chromium-review.googlesource.com/640691Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47684}