GitLab

- Avoid reference/pointer into std::vector.
- Just use back() and pop_back() as needed.
- Remove some local variables that are only used once or twice.

Change-Id: Ideaf1dfdbbd9fe6091af87e7d72932d3eb2272af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585172Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654618}