Bug 904197 - Use the union of continuations' rects in sticky positioning calculations. r=dholbert

Reftest inline-3.html fails because handling the sticky element's margin correctly will take more work; similarly, border/padding on containing-block continuations won't be handled quite right. Reftest column-contain-1a fails because some of the anonymous blocks inside an nsColumnSetFrame have 0 height.
Bug 904197 followup: make reftests use Ahem font, to avoid pixel-alignment difference between block vs. inline elements. (test-only)

--HG--
rename : layout/reftests/flexbox/ahem.css => layout/reftests/position-sticky/ahem.css
Bug 916939 - Drop support for named and indexed access on cross-origin windows. r=bz