[ios] Tab strip UI polish.

Bug: 857431
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I1c8ae739deab88470f40e5b3c2e241406ecb233e
Reviewed-on: https://chromium-review.googlesource.com/1138797
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575661}
2 files changed