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When using the thumb strip, when the user is in the tab grid, they can
swipe the BVC up from the bottom of the screen to go back to it at any
time. Because of this, when switching tab grid pages, the BVC at the
bottom of the screen has to switch as well.

In general, the idea is that the BVCContainerViewController will always
be present. In the tab grid, if the user is on a page with a tab, it
the container will have the appropriate BVC. If they are on a page
without a tab (Recent Tabs or incognito with no open incognito tabs), it
will have no BVC but still be onscreen, ready to be filled.

All of the tab grid/scene controller presentation methods now take an
additional argument of whether to dismiss the tab grid after activating
the given Browser/UIViewController. This allows the tab grid to request
changes to only the active BVC without leading to dismissal of the tab
grid.

Bug: 1094335
Change-Id: I876ccaf01416caccc3bb85cccc5701527a45840a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577462
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837059}