Remove TabModelObserver usage from BVC

Replace usage of TabModelObserver's didChangeTab with WebStateObserver's
didStartNavigation.


Bug: 911350
Change-Id: I35125a96a053e2ed4f27bfadfcb5a732d33d807a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630231
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665530}
2 files changed