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The Autofill Assistant stack's lifecycle is not using Chrome's framework
designed to provide exactly that. This patch introduces a TabHelper that
will become a proper entry point to control construction and destruction
of components of the Autofill Assistant stack that depend on a WebContents.

This initial patch only adds an empty TabHelper and test and hooks it up in the
tab_helpers registry behind a default disabled AutofillAssistantTabHelpers
feature.

Bug: b/169389571
Change-Id: I3129fd04e5e4dd6c65c1df891383cf29cad508d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521620
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825719}