SyncFS: Prepare for passing different LocalChangeProcessor based on origin (app)

- Add SetLocalChangeProcessorCallback(), which takes a callback that returns
  LocalChangeProcessor (== usually a RemoteFileSyncService) for a given origin
  (so that we can start returning v2 backend for a certain origin)
- SetLocalChangeProcessor() is also left to make the existing tests pass,
  with a TODO comment to deprecate either one of the methods.

BUG=324215
R=tzik,nhiroki

Review URL: https://codereview.chromium.org/94603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237783 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237783