In all copyright messages (and in the Xcode project organization name) standardize on "The Flutter Authors", adding "The Chromium Authors" to the Flutter AUTHORS list. This reduces inconsistency in the copyright lines in this repository, moving closer to a single consistent copyright+license (as in flutter/engine and flutter/flutter)
Updates the validation script to no longer accept "The Chromium Authors" or "the Chromium project authors" in first-party code.
Adds a new CI check that all code files have a copyright+license block (and that it's one we are expecting to see).
Fixes the ~350 files (!) that did not have them. This includes all of the files in the .../example/ directories, following the example of flutter/flutter. (This does mean some manual intervention will be needed when generating new example directories in the future, but it's one-time per example.)
Also standardized some variants that used different line breaks than most of the rest of the repo (likely added since I standardized them all a while ago, but didn't add a check for at the time to enforce going forward), to simplify the checks.
Fixesflutter/flutter#77114
