please allow users to enable hw-acceleration on linux

[pdknsk]

This is an old and well-starred bug unfortunately closed for discussion.

https://code.google.com/p/chromium/issues/detail?id=137247

It's not available for what I think is a poor reason, spelled out in a recent bug report.

> Today we don't have a Linux GPU decode owner, so we have chosen not to activate something which we have no one to support and may have to break arbitrarily in the future for ChromeOS.

https://code.google.com/p/chromium/issues/detail?id=463440

This should be an unsupported option at least, for users who choose to ignore the GPU blacklist. For Intel hardware, it could be made default today I'm sure. With 4K added to YT recently, it's necessary.

[pdknsk]

This is particularly, well, stupid IMO because encoding is hardware accelerated, if chrome://gpu is right.

Video Decode: Software only, hardware acceleration unavailable

Video Encode: Hardware accelerated