Threading clean-up for component updater differential update.

This cleans up a threading model oversight - methods in the differential
update operation should only ever be called on the SequencedTaskRunner, never
on the IO thread itself.

BUG=304879

Review URL: https://codereview.chromium.org/221343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@261062 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed