IPH: Overrides button outline color to white

Overrides button outline color to white as requested by UX. Currently
the outline is black under dark mode.

Bug: 1146259
Change-Id: I9d5a04bca5f094282b9e51d5f17aae08486b2b3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522226


Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824923}