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This tests what happens when a new flow gets started in a new tab.

It also fixes a bug where:
Flow A started on Tab A, Flow B started on Tab B, switching from B to A,
since B was attached, the |Attach| was not called and B was shown on
Tab A.

The CL also introduces some safety null checks.

Bug: b/152967053
Change-Id: I9789e823fb4b1c129db35504484215ea50e1b7ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135746Reviewed-by: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#756671}