- make the ScrollBehavior instance long-lived, rather than recreating
  it each time we update the list contents.
- have OverscrollBehavior track the total height of the contents and
  the height of the scrollable region, so that it can determine when
  to stop scrolling down.
- teach OverscrollBehavior about how to determine when to stop
  scrolling down, and how to bounce when it's too far down.
- replace the 'energy' concept in Particles with a method that sets
  the energy and direction at the same time, instead of assuming that
  the direction is always positive when setting energy.
- make FixedHeightScrollable lists track the number of items in the
  list and have them update their ScrollBehavior regarding this
  information as it changes.
- track how many items are currently showing in the list stock list.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1097373002