[SyncFS] Add UninstallAppTask

 * Adds the out of class implementation of SyncEngine::UninstallOrigin
   and its wiring up code.

R=kinuko, nhiroki
BUG=240165

Review URL: https://codereview.chromium.org/25839002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228246 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228246