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.
