[ios] web_view_shell: Set location bar's borderStyle to make its corners round

This simplifies the existing code while achieving basically the same

some padding to the left, we can simply set |_field|'s borderStyle to
UITextBorderStyleRoundedRect.

result: instead of fiddling with a CALayer's cornerRadius and adding
Change-Id: I9310ddc1d17f40d77c079de69fcd7d2f62182930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6174658
Reviewed-by: John Wu <jzw@chromium.org>
Auto-Submit: Raphael Kubo da Costa <kubo@igalia.com>
Commit-Queue: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1406439}