Implement simple bottom toolbar with MVC framework

This change implements a simple bottom toolbar that has an android view,
a composited component, resizes the viewport, and scrolls on and
off-screen. This toolbar is implemented using the new MVC guidelines:

- The BottomToolbarController sets up all the components of the bottom
  toolbar.
- The BottomToolbarMediator is responsible for pushing updates to the
  model and running most of the business logic.
- The BottomToolbarModel is a blob of state with no actual business
  logic.
- The BottomToolbarViewBinder reacts to events from a
  PropertyModelChangeProcessor and updates the relevant views.

BUG=815324,825965

Change-Id: I75e5aac6a2bb2a21ea9e4010fc81076d2d39da8b
Reviewed-on: https://chromium-review.googlesource.com/980577
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548436}