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This allows the client to handle backing out of an error page, such as
an SSL interstitial, when there's no navigation entry to return to.
This would cover the case where the first page that a tab navigates to
has an SSL error.

The implementation for the Android shell simulates the user pressing
'back'.

Bug: 1022607
Change-Id: Id05e035d75b124b63c09c074b38a78eb1810e8e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910869Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715287}