If ReassociateLocalTab is called with a tab node that is already mapped to a
tab, it's possible to wind up with the synced session tracker holding two
tab objects referring to the same tab id. This can lead to memory corruption.

This CL makes the tracker defensive against that scenario, and adds some more
data verification in the unit tests

BUG=714524, 639009

Review-Url: https://codereview.chromium.org/2856913007
Cr-Commit-Position: refs/heads/master@{#469553}