GitLab

- Log all blocked or allowed attempts.
- Export API to drain log, and to register for log notification.
- New tests.

BUG=769590
TEST=chrome_elf_unittests.exe, Whitelist*

Change-Id: I4f1ec911efbe3eada231aa36b0fadceb3756521e
Reviewed-on: https://chromium-review.googlesource.com/847894
Commit-Queue: Penny MacNeil <pennymac@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532643}