[Sync] Move StartAssociation from NonUIDataTypeController to SharedChangeProcessor.
This CL copies a utility function, BindToTaskRunner, from //chromecast/base.
I discussed adding it to //base with danakj@ but apparently it has been
rejected before.
BUG=
638771
Committed:
https://crrev.com/510f3e9bff01537bca7799b3e9cc2142d64f7376
Cr-Commit-Position: refs/heads/master@{#413175}