ce.BranchNode: Insert slugs based on CE children length, not DM

Due to Firefox' Ctrl-A creating an outer selection (0,len) as
opposed to inner select like in Chrome (1,len-1), deleting leaves
the document completely empty (with no cursor anywhere in the
document).

However, setupSlugs() wasn't inserting a slug because the internal
list still existed (so this.model.length !== 0).

Using this.$.children().length instead.

Bug: 50947
Change-Id: I1517ebcc5b31e2544559b851174482b0c872b24b
Merge "ce.BranchNode: Insert slugs based on CE children length, not DM"