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This is achieved by flattening the existing AssistantWebView into
AssistantWebContainerView.

Note that a follow up CL will take the name 'AssistantWebView' and
reassign it to what is now called 'AssistantWebView2'.

Bug: b:146520500
Change-Id: I242085c039317936047341193ba05c857a144182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040199Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#738778}