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This allows querying the CanGoBack status of the underlying app

Bug: b/145762033
Test: None
Change-Id: I330d7124dd7817db69f5ff41bfa58f6059971113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008066Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733446}