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This CL also makes some improvements to InfobarContainerCoordinator so
the test can pass:
- isPresentingInfobarBanner is now directly based on the ChildCoordinator
Banner presentation and not just the existence of a VC.
- A disconnect method is added to make sure the mediator stops observing
WebStateList before being dealloced (This is a common pattern throughout
mediators since the memory de-alloaction order in unittest can cause this)
- Adds a legacyContainerFullscrenSupportDisabled to disable FullScreen
support in the legacyContainer. This is needed to avoid a crash after
cleaning up each test case, in order to properly fix this we'd need to
know when the legacyContainer has been dismissed, that would require
some refactoring and the effort/risk is not worth it at this point.
- Removes a DCHECK in the legacyContainer since now the count of total
Infobars doesn't necessarily match the count of total Infobars in the
legacy Container. This because one of them can be a message. The iOS
implementation of the container was merely sending infobar.count() for
position, meaning we were never using position anyways.

Bug: 911864
Change-Id: Ie46c9dc88d2f5277401eb1d08aa54a09d4dbaf82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566756
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652604}