WebUI: Convert --cr-dialog-wrapper mixin to ::part(wrapper)

Bug: 973674
Change-Id: I2d9971b41bd2f399914a6ab57c995cb5f1ca383b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662730
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670260}
4 files changed