chromium: Support for hardware accelerated video encoding/decoding

@selfisekai

First of all, thanks for shouldering this herculean task of maintaining the Chromium browser. I would like to enable VA-API hardware accelerated video encoding/decoding support on my laptop, which is a Dell with Core m3-6Y30 processor. With Firefox, video playback is handled by VA-API with no issues, but on Chrome, even with the flags documented here, namely

--use-gl=angle --use-angle=gl \
--enable-features=VaapiVideoDecodeLinuxGL,VaapiVideoEncoder,VaapiOnNvidiaGPUs \
--ignore-gpu-blocklist --disable-gpu-driver-bug-workaround

Chromium still makes no use of VA-API. There is no error reported with VA-API initialization. chrome://gpu reports Video Acceleration Information as completely empty. Video Decoder Name shown in chrome://media-internals is still "FFmpegVideoDecoder".

Do you have any idea about how this could be made to work? Any suggestion is appreciated. That page also mentioned:

To support proprietary codecs such as, e.g. H264/AVC1, add the options proprietary_codecs = true and ffmpeg_branding = "Chrome" to the GN args (please refer to the Setting up the build Section).

Do we need these build flags?