Ensure ExtensionService exists before AppListSyncableService::BuildModel

BUG=334963
R=benwells@chromium.org, tapted@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/148383013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248756 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248756