Branching Perfetto for Chrome milestones

Merging a (set of) Perfetto change(s) to a Chrome milestone release requires creation of a branch in the perfetto repo, cherry-picking of the change(s) to the branch, and updating the DEPS file in Chrome's milestone branch to point to the new perfetto branch's head.

Creating the perfetto branch

1. Determine the branch name: chromium/XXXX, where XXXX is the branch number of the milestone (see Chromium Dash). Example for M87: chromium/4280.

2. Check if the branch already exists: if yes, skip to cherry-picking. To check, you can search for it in https://github.com/google/perfetto/branches.