We recently accidentally broke the "Enable DRM" prompt without realising, so
this adds a test that if EME is disabled and is attempted to be used we we
prompt the user to enable EME.
It is important we don't regress this feature as without it users with DRM
disabled (which is our Linux popuation by default) may try to playback DRM
protected video and it won't work and there won't be an obvious way to make it
work.
MozReview-Commit-ID: D4X7D4wkwRy
--HG--
extra : rebase_source : d0c7bf45c0a3b4ffb517a3b7c008230bd750fc8d
