[SyncFS] Prioritize App management task on SyncEngine

R=kinuko, nhiroki
BUG=240165
NOTRY=true

Review URL: https://codereview.chromium.org/105013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239454 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239454