[ios] Don't set the WebStateList active index to an invalid value.

Invalid indexes returned by -DetermineNewActiveIndex can put WebStateList in a
bad state.  Don't allow these values.

Bug: 877792
Change-Id: If996221049382d9ee911971ed974eebe8a4411de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600832
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657814}
2 files changed