[SAML][OOBE]: Replace webview border radius with solid border

Bug: 1286396
Change-Id: Iebe981d52bfef936cf20f407e2dc6db1c6ce5440
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3378670


Reviewed-by: Roman Sorokin <rsorokin@chromium.org>
Commit-Queue: Danila Kuzmin <dkuzmin@google.com>
Cr-Commit-Position: refs/heads/main@{#958201}