Bug 853831 - Fix fixed margins on the opposite side of overscroll. r=kats

If overscroll eats into a fixed margin, we need to apply the opposite
offset to the opposite side of the axis this occurred on. This has the effect
of fixed-position elements aligned to the bottom of the screen remaining
visible when at the top of the page.
Bug 848719: URL Bar corruption with dyn-toolbar. [r=mfinkle]

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extra : rebase_source : eec4e124028c43af2d1b237c14e711d883417b89