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

[e1526425]

I just tested it: When applying the patch, you need to add 
"--disable-accelerated-video-decode" to default flags, as the present 
default flag "--ignore-gpu-blacklist" seems to enable hardware 
accelerated video decode (according to about:gpu).

So please consider adding the patch and also set the 
"--disable-accelerated-video-decode" flag by default so that people do 
not run into problems.