Bug 891056 - Support "snapped view" in Windows 8.1. a=lsblakk

Roll-up of patches 2-4.
Bug 918833 - Improved profiler thread register. r=ehsan, a=lsblakk
Bug 919090 - Let content processes use sigaction and tgkill if profiling enabled. r=kang, a=lsblakk
Bug 921522 - Use already-established flex container content-box main size to determine packing space, instead of checking reflow state. r=mats, a=lsblakk

--HG--
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1a.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1b.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1a.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1b.xhtml
Bug 921770 - Avoid doing things with js/src from top-level when building --with-libxul-sdk. r=gps, a=lsblakk
Bug 921822 - Use existing install manifest for make install in js standalone builds. r=gps, a=lsblakk