GitLab

Sorts sources blocks as part of gn format. This ended up being more
complicated than I expected because of the subtlety of header comments,
and because no other formatting reorders expressions at the moment.
But, there seems to be quite a lot of unsorted lists per below, so I
guess it's a good thing to have.

Application to tree at https://codereview.chromium.org/960413003.

R=brettw@chromium.org
BUG=348474, 456014

Review URL: https://codereview.chromium.org/962003002

Cr-Commit-Position: refs/heads/master@{#318580}