Fix logic that selects a tab when one is closed.
Fix logic that selects a tab when one is closed.

We don't actually care whether the selected tab can focus (if it
can't, switch_to is graceful, and we don't have any special handling
to look for another tab anyway), so instead just validate that the
selected index exists (which it might not).

Change-Id: I686a5d5756018342d0abc446692367474edeb8ca

jmorse/cyanide,jmorse/cyanide,jmorse/cyanide,jmorse/cyanide