Sync the launch type pref for apps.

This CL syncs the launch type pref for apps by adding a property to
app_specifics.proto. The sync data is processed in ExtensionSyncService.

extensions::SetLaunchType has been changed to take an ExtensionService
so that it is able to sync the pref change when a launch type is updated.

A value has been added to the LaunchType enum to represent an invalid
launch type preference.

BUG=181576
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/93883004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244112 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244112