- 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
ebc879babf