Bug 1387084 - Use instant scroll behavior when doing pixel scrolling. r=Gijs

Touchmove and wheel events are sent frequently enough that smooth scroll behavior
prevents the expected pixel distance from being reached before the next event.

Also replace aSmoothScroll parameters with aInstant to better reflect how this
works: its purpose is to force instant scrolling, whereas omitting it falls back
to "auto" (which may mean instant or smooth depending on different factors).

The ensureElementIsVisible call from browser-customization.js can go away as
customize mode doesn't add padding around the window anymore.

Finally, remove the unused scrollPosition and scrollPaddingRect properties.

MozReview-Commit-ID: 3Ac7g6zZ0hW

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extra : rebase_source : e43d0bcab82c74e65d01a0fd79bfaec96952b35a