Bug 841445 - Add a 'mach run' command to launch the compiled binary [r=gps]

DONTBUILD (not part of the build)
Bug 841445 - Make 'mach help <command>' call print_help directly [r=gps]

DONTBUILD (not part of the build)
Merge m-c to inbound.
Bug 851828 - Type safety improvements for the profiler in Win64 builds. r=benwa
bug 828508 - use higher-res favicons for tab titles in hidpi mode if available. r=dao
Bug 849253 - Followup to really unbreak BE archs: use USE_SKIA_GPU in HelpersSkia.h too r=gwright