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.
• Modified Web platform tests DOM: Test {mouse,}wheel and touch{move,start} events are non-cancelable when there are no non-passive event listeners web-platform-tests/wpt#34464

Implementation commitment:

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

[garykac]

This reference to "mousewheel" seems out of place since it is not mentioned anywhere else in the specification.

If we want to include this, we should add mousewheel to Section 9 "Legacy Event Types" with the other deprecated event types. I've created an issue to track that: #331

[zcorpan]

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

[garykac]

Thanks!