Extract bug numbers noted "Bug: 99999" in addition to the older
"BUG=99999" style.

Bug: None
Change-Id: If29139ee2bae3c515748395a846e45ce6995d9ec
Reviewed-on: https://chromium-review.googlesource.com/548718Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>