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This CL places a root container at the root of the view controller hierarchy,
instead of the tab grid. The root container has a content view controller
that can be swapped between regular mode or incognito mode.
This refactor is required to enable switching to incognito mode.

BUG=686770

Review-Url: https://codereview.chromium.org/2800313002
Cr-Commit-Position: refs/heads/master@{#464420}