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This CL splits StackLayout into a base class and child class:

- StackLayoutBase is an abstract base class that contains most of the existing
  logic.

- StackLayout is a non-abstract child class that contains the pieces of
  existing logic that are assume there will always be exactly two stacks: one
  for all normal tabs, and one for all incognito tabs (the incognito stack is
  not displayed if empty).

This enables us to prototype new UIs on top of the tab switcher. I've already
built one prototype on top of this base class; we may end up needing to tweak
StackLayoutBase slightly as we build different types of prototypes, but I think
this is a pretty good start.

Bug: 648314
Change-Id: I25ca7169a1dfd3e780adea58ecb8043da9b55035
Reviewed-on: https://chromium-review.googlesource.com/957875Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543165}