This CL adds support for updating the list of disabled tokens, via the
existing component updater. Like other origin trials component updates,
the list is plumbed through to the ChromeOriginTrialPolicy via prefs
and the browser command line.

For now, the tokens will be pushed down as a list of base64-encoded
token signatures. Next step is to use a bloom filter, to be more
space-efficient (as in the design doc).

BUG=582042

Review-Url: https://codereview.chromium.org/2733023002
Cr-Commit-Position: refs/heads/master@{#457547}