Indicate whether a touch event will cause scrolling

Indicate whether a touch event will cause scrolling

With the eager GR, we know the default actions of a touch event before
it is forwarded to a given target. For now, flag each touch event to
indicate whether it will cause scrolling (if unhandled).

This is particularly useful for decoupling the redundant slop region
checks we have in the code to try to guess whether a given touch
sequence has started or may start scrolling.

BUG=425586, 432605