Bug 894746 - Properly truncate mach build terminal footer; r=ted

DONTBUILD (NPOTB)
Bug 895962 - Dismiss the find bar when another app bar appears [r=rsilveira]
Bug 896028 - Add a standard Makefile variable for mochitest-metro files [r=gps]
Bug 889699 - Make gstreamer backend return more accurate results for canPlayType() r=alessandro.d
Bug 895670 requires a clobber
Merge inbound to m-c.
Merge inbound to m-c.