• Add a pref to gate special clearkey key systems.
• Add a new key system that will be used to configure the clearkey CDM.
• Change Gecko code to handle the new key system (if the pref is set).

I do not plan to change the clearkey CDM in this bug, this bug is focused on the new key system and plumbing it. Following this, I'll rework the clearkey CDM to behave differently if it's created with the key system added here.

This allows us to use the EME_KEY_SYSTEM_CLEARKEY_WITH_PROTECTION_QUERY from JS.
This is done by changing the code in two places

• We consider the key system as clearkey in IsClearkeyKeySystem if it is preffed
  on. This will allow our checks that take place as part of granting key system
  access to pass.
• We include the new key system in the tags supported by the clearkey CDM. This
  means that when Gecko searches for a GMP that supports the new key system, the
  clearkey GMP will be considered as a match. This functionality is not gated
  by pref for two reasons: 1) the gating done elsewhere is sufficient to prevent
  usage if the pref is not set, 2) the loading of GMPs is not re-done if we
  toggle the pref, so if we gated this functionality behind a pref we'd need to
  trigger some kind of reloading of GMPs when the pref is toggled.

Depends on D122143

This exposes a new key system behind a pref. The new key system doesn't do anything differently than clearkey yet.

Testing:

• Set media.clearkey.test-key-systems.enabled to true.
• Navigate to this test page I've lovingly crafted.
• Enter org.mozilla.clearkey_with_protection_query in the text field and hit the button.

If the code works correctly then the video should play all the way through.

https://hg.mozilla.org/mozilla-central/rev/cee45dd2cfb9
https://hg.mozilla.org/mozilla-central/rev/9bc9be04ebbb