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Previously the host for Assistant Settings used the Content Service
for embedding WebContents, but this introduced a number for focus and
A11Y related bugs.

Now, we are migrating back to a WebView based implementation that
doesn't exhibit the same buggy behavior.

Manual tests run:
- Tab traverse in the forward direction
- Tab traverse in the backward direction
- ChromeVox traverse in the forward direction
- ChromeVox traverse in the backwards direction
- Changing between windows and then tab traversing
- Changing between windows and then ChromeVox traversing

Bug: b:146351046
Change-Id: I02601da8c8d726faaad527dda5785b4ccc60102b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999042Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#731770}