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The NonBlockingDataTypeController tracks the sync state of
a non-blocking data type and sends signals to its components from the UI
thread.  It is responsible for enabling and disabling sync for a data
type according to user preference and the availbility of the sync
backend.

This CL also includes some changes to the NonBlockingDataTypeProcessor,
which is currently just a stub of what will eventually become
non-blocking sync's model thread component, so that it can be used in
testing NonBlockingDataTypeController.

BUG=351005

Review URL: https://codereview.chromium.org/249843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266344 0039d316-1c4b-4281-b951-d872f2087c98