[SyncFS] Move sync_file_system/sync_task* to drive_backend directory

 * Move SyncTask related files to drive_backend directory,
 * Move related classes to drive_backend namespace,
 * Fix lint failures

BUG=344769

Review URL: https://codereview.chromium.org/196723005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256793 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256793