Bug 850534: Various Yarr performance improvements, r=dmandelin
Bug 850889 - Fix animation conflict with overscroll and dynamic toolbar. r=kats

This fixes the conflicting animations when the dynamic toolbar is hiding/showing
and overscroll is snapping back simultaneously. This is by not clamping the
entire viewport on margin-setting, and by making sure that only calling
setFixedLayerMargins changes the fixed layer margins.
Bug 850889 - Fix jank when animating dynamic toolbar. r=kats

Only update the Gecko-side fixed layer margins when the view is fully visible
or hidden to avoid jank.
Bug 851897 - Fix crash in mozilla::widget::nsWinMetroUtils::GetActivationURI. r=jimm