[Pkg-chromium-maint] Bug#856255: chromium: Enable "Hardware-accelerated video decode" flag

[Esokrates]

As I have noticed now, these patch has been applied once:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=793815
but was shortly after disabled again:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=804901

I think completely disabling the code path was not the right decision,
as the patch only enables something which could be disabled by a flag, 
just look at the "IsAcceleratedVaapiVideoEncodeEnabled" bool in the patch.

Users with broken acceleration should furthermore disable the gpu 
override list (maybe disable it by default).
Apart from that we could additionally pass the 
"disable-accelerated-video-decode" flag by default, so the patch is 
always disabled for sure.