Figure out what to do with OpenH264 (downloads) in Tor Browser

We should think about what we want to do with the OpenH264 video codec plugin which Firefox downloads since version 33 shortly after it gets started for the first time (see: http://andreasgal.com/2014/10/14/openh264-now-in-firefox/ and https://wiki.mozilla.org/QA/WebRTC/OpenH264).

The good news is, the code is free: https://github.com/cisco/openh264. The bad news is it needs to get downloaded from Cisco as a binary blob due to patent issues. And there is currently now known way to build this binary blob deterministically: https://bugzilla.mozilla.org/show_bug.cgi?id=1115874.

I think we should make sure that the plugin does not get downloaded as:

1. It is currently only used for WebRTC which we have disabled (https://bugzilla.mozilla.org/show_bug.cgi?id=1150544#c8) (we should make sure that this argument still holds for ESR 38 when we ship it if that matters)

2. The binary blob is not built reproducibly which poses security risks. Although there seems to be kind of a mechanism for Mozilla to verify things:

Mozilla and Cisco have established a process by which the binary is verified as having been built from the publicly available source, thereby enhancing the transparency and trustworthiness of the system.

3. The download uses essentially Mozilla's "cert pinning". We might want to have something stronger in place.

...

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=769716 for a way to disable the plugin download.