Gitwatch supports closing bugs with a FIX(ES|ED|ING)? tag in the
commit message, but we only check BUG to ensure a bug is
specified, add the fix fields to BugsFromDescription so that presubmits
will still pass if only eg: FIXES=b:1234 is given.

Gitwatch reference:
  http://shortn/_LGPgBjgi3A

FIXED=b:203812728
TEST=presubmit_unittest.py doesn't fail more than it already does

Change-Id: I6afc38c786e281dcefb4d359bb9212ebe5980ed6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3237598
Auto-Submit: Sean McAllister <smcallis@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>