Update some geometry logic 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 the necessary updates to the geometry logic. After
this CL (and the preceding ones), the only thing keeping StackLayout from
displaying more than two tab stacks is that a lot of the logic is hard coded to
always show all normal tabs in one stack and all incognito tabs in another. I
will split much of StackLayout into a base class in a future CL to allow
creating variants with different behavior.

When we display two tab stacks, we use asymmetric margins: we put one tab stack
up against the edge of the screen (with a small border added by the Stack
itself), and show the edge of the other tab stack on the other side. This CL
does not change the behavior for this case. For three or more stacks, the
behavior I'm going with for now is to make the margins symmetric. So no matter
which stack is focused, we show the current tab centered, with the edge of a
neighboring tab showing on either side if present, or a blank margin on that
side otherwise.

Bug: 648314
Change-Id: I383d9601f521a2eec47263f219027bc23818b3e2
Reviewed-on: https://chromium-review.googlesource.com/940613


Commit-Queue: Ryan Landay <rlanday@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542201}