[roll-dep] Use Bug: instead of BUG=
[roll-dep] Use Bug: instead of BUG=

This is the more modern syntax and everything should already work with
it.

R=tandrii@chromium.org

Change-Id: I7acbe6cc7c663e03e9a72c2b6c155a7982477146
Reviewed-on: https://chromium-review.googlesource.com/c/1289233
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Reviewed-by: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools