GitLab

In embedded Assistant UI experiment, we will move the web view to its
own container.

This cl is the first part to add the container. A follow up cl will
handle the back arrow.

Bug: b/141268413
Test: new unittest
Change-Id: I9d3b9663b7a60f02e9f50e0076d9c342c299d8f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837294
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707946}