Prevent JS errors when surfaceReady fires after deactivation

If you clicked "Read" while the editor was loading
(but only while the "Edit" tab was already active, not before)
then you could get in a situation where surfaceReady fired
on an already-aborted target, which caused JS errors.

It seems like we should clean more things up in this
case, but I don't know what they are. In any case,
we should not try to set things up on a non-activating
target when surfaceReady fires.

Change-Id: Id57bd63ff288156725e472e7d89009022090253a
Merge "Prevent JS errors when surfaceReady fires after deactivation"