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Previously, the back button immediately navigated backwards in the
Assistant container's UI state.

Now, the back button will navigate backwards in the WebContents history
stack. Only when there is no more history do we return control to the
Assistant container for handling.

See bug for demo.

Bug: b:112355878
Change-Id: I943407c85497f09a5360f21353301b6bc6823315
Reviewed-on: https://chromium-review.googlesource.com/1167646
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582736}