Specify non-cancelable wheel events when there are no non-passive listeners

[zcorpan]

Fixes #282

This uses the same language as in https://w3c.github.io/touch-events/#cancelability

The following tasks have been completed:

• [ ] Confirmed there are no ReSpec/BikeShed errors or warnings.
• [x] Modified Web platform tests https://github.com/web-platform-tests/wpt/pull/34464

Implementation commitment:

• [x] WebKit already implemented
• [x] Chromium already implemented
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=) (not sure if Gecko implements this)

[zcorpan]

I've removed mousewheel for this PR. It can be added back as part of fixing #331

[garykac]

Thanks!