[iOS] Factor web params creation into a tab utility.

This CL creates a tab utility file (free functions used in managing
tabs), and adds a function to it to create web navigation params from
a URL, transition type, and post data. It factors this code out of BVC.

Additionally, it moves the logic for translating a tab model index of
NSNotFound into the last index of the model into TabModel. This was
also in the BVC.

Bug: 903338
Change-Id: I856163785d34bb064ea27b76c3094834d3f31626
Reviewed-on: https://chromium-review.googlesource.com/c/1326509
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607548}
8 files changed