[exp] Browser-side fling.

[exp] Browser-side fling.

When a FlingStart event comes in, start generating a sequence of ScrollUpdate
events, and send those to the renderer instead of the FlingStart event. This
moves the control of the fling scroll entirely in the browser process.

Details:
 . The FlingStart event reaches InputRouterImpl. Instead of forwarding this
   event to the renderer, the input router replaces it with a sequence of
   ScrollUpdate[WithoutPropagation] events. The first ScrollUpdate event is
   generated and dispatched immediately.
 . The subsequent ScrollUpdate events are generated hen the ACK for a previous
   ScrollUpdate[WithoutPropagation] event is received.
 . The location of all the ScrollUpdate[WithoutPropagation] events are the same
   as the location of the FlingStart event.
 . The fling is cancelled when a FlingCancel is dispatched, or when the fling
   velocity slows down to zero.
 . The fling is cancelled when an overscroll notification comes in from the
   renderer.
 . The fling is cancelled when a ScrollUpdate[WithoutPropagation] comes in as
   un-consumed (and the scroll-update was sufficiently large, since towards the
   end of the fling, individual scroll-updates can be too small to cause any
   scrolls, and can thus be erroneously acked as un-consumed).

TODO:
 . Make sure the fling is stopped at the right times:
   - when sending a key event,
 . Flywheel
 . Aura overscroll
 . WebView sync

BUG=TBD

[sadrul, 2014-09-26 19:07:03]

Patchset #8 (id:140001) has been deleted