[Autofill Assistant] Run Autofill Assistant in a normal tab.

Before this patch, Autofill Assistant would only run in a Custom Tab.

With this patch, Autofill Assistant can run in a normal tab. The UI is
re-created when switching to a tab that has Autofill Assistant running.

In practice, since Autofill Assistant can currently only be started from
a Custom Tab, this means that an Autofill Assistant session can now
follow a tab as it moves out of Custom Tab (by selecting the menu Open
tab is Chrome.)

The approach is described in go/aa-any-tab Still missing: reusing UI
instances when switching from a tab with Autofill Assistant to another
tab with Autofill Assistant.

Bug: 806868
Change-Id: I8741c6f2d40d39a9e10d7526721d932608dea2c4
Reviewed-on: https://chromium-review.googlesource.com/c/1458178
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630330}
12 files changed