Reduce scroll performance impact of wheel listeners

[RByers]

wheel events are a big problem for scroll performance. Eg. using timestamps to monitor latency @n8schloss has found that wheel listeners are a real problem for scroll performance in Facebook. There are a number of incubation-style efforts underway to try to improve this, which if successful should probably make it back into the UI Events specification somehow.

In particular:

• When all wheel listeners are passive, wheel events should be uncancelable (already shipping in Chrome and I believe Safari and Firefox). Eg. perhaps UI Events should have a "cancelability" section like Touch Events does
• Define touchpad "latching" and use that to make wheel events uncancelable during a scroll
• Let developers opt-out of scrolling bubbling up past some elements (eg. fixed position), eliminating the need for some non-passive wheel listeners
• Define a declarative wheel-action API to make it easier to move to a passive-by-default model as we're working towards for touch.

/cc @tdresser @dtapuska @igrigorik