[SyncFS] Add priority supported SyncTaskManager::ScheduleSyncTask

BUG=240165
NOTRY=true

Review URL: https://codereview.chromium.org/109873003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239453 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239453