Bug 836877 - mach now errors if executed with Python 3; r=ted

DONTBUILD (NPOTB)
Bug 794162 - Mach command to generate a .clang_complete file; r=ehsan

DONTBUILD (NPOTB)
Merge the last PGO-green inbound changeset to m-c.
No bug, Automated blocklist update from host bld-linux64-ec2-661 - a=blocklist-update
Bug 788185: add a/v sync to Audio/Video Conduits r=derf
Merge mozilla-central into mozilla-inbound