PointerEvent: set pressure to 0 unless pointer is down

[appsforartists]

Nonsensically, pointerup events currently have a non-zero pressure on iOS. As outlined in w3c/pointerevents#146, the spec should be updated to make it clear that if a pointer is up, pressure must be 0. (This is already the case for pointers that don't report granular pressure - this proposal only makes that rule consistent.) This PR also makes PEP's behavior consistent with Firefox on all platforms.

https://github.com/w3c/web-platform-tests also needs to be updated, both to assert that pressure is 0 in pointerup and to test for custom pressure in a pointermove; the current test checks in pointerover.

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[scottgonzalez]

Google, Inc. is not an author.

[appsforartists]

@willnorris - thoughts on AUTHORS? Apparently, when Polymer maintained this, they didn't list Google either.