Bug 1229125 - Correct velocity computation for pan gesture events. r=botond

When scrolling down, panInput.mLocalDisplacement.y will be positive, so
aAdditionalDelta will be positive, and newVelocity must be positive.

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extra : commitid : GlT0IqOWqiI
extra : rebase_source : 2fc0347d943ac7bf8eac2f2da6f7b6b217bdbe0d