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- 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