Add update-w3c-deps script to Tools/Scripts
This is a meta-import script that is responsible for doing all the
git shenanigans to import new versions of the W3C Test repos. It uses import-w3c-tests to do the heavy lifting, but also does all of the git manipulation necessary to ensure clean version history.
This patch also adds a 'W3CImportExpectations' files that lists which directories should be ignored during an import (i.e., it acts as a blacklist, so that we will otherwise pick up new changes automatically).
Note that this script does *not* automatically push anything to a source repo. The user still needs to verify that the import works, update any TestExpectations suppressions, and push the repo changes back to the Blink mirrors of the W3C repos.