GitLab

While looking at mojom files I'm nominally the owner of, I realized
that one had become redundant.  So, I just deleted it and checked the
other to make sure it had the right 'nullability' on the method args.

Also moved around code to build against the de-duped mojom file.

BUG=407673
TEST=build with gyp and gn

Review URL: https://codereview.chromium.org/503363004

Cr-Commit-Position: refs/heads/master@{#292054}