Implement fetch image helper

We would do that algorithm in chrome process, then select a image for the virtual media control interface.

https://w3c.github.io/mediasession/#fetch-image-algorithm

Could we stop at point 4 and just pass the src URL to the Platform?
I don't know about Mac OS X and Android, but for Windows and Linux we can safely pass such an URL.

That way we could shift all of the responsabilities down to the Platform.
Otherwise we'd need a temporary directory which needs to be cleaned up, maybe even needs a maximum size limit etc.
Specifically for MPRIS, where we would download a file just to build up a file:/// URL again.

This is maybe something to consider there, UNLESS we ever want to display the artwork within the user agent of some sort.

That is just a recommended way, and I think we won't follow that 100%. This bug is filed for a purpose to ensure that we would feed the event source with artwork url.

Per bug1623971 comment9, we have to fetch the image inside Gecko, instead of letting external framework to load the URL directly.

We will implement a fetch-image-helper in this class, then the platform event source can selecte the preferred-size artwork and get the image by using this helper.

The notification of the reply [1] on Phabricator is easily to miss, so I also set NI here. Thank you.

[1] https://phabricator.services.mozilla.com/D69288#2114435

A quick update, I found a memory leak issue in the imgTool so I spent some time on that, and will solve that issue in bug1628929 before fixing this bug.

https://hg.mozilla.org/mozilla-central/rev/645373573077