Straightforward generalizations in StackLayout to support more than two stacks

I have a series of CLs in progress to allow StackLayout to support more than two
tab stacks. This CL contains easy generalizations (e.g. adding loops) of
existing logic that currently assumes there will always be exactly two tab
stacks.

Updating the geometry logic is more involved, so I will do that in a separate
CL.

Bug: 648314
Change-Id: I8613cdd5ec345dfa868d992575c5d8d560ca5b04
Reviewed-on: https://chromium-review.googlesource.com/940570


Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542181}