Bug 1536102 - P2: Add encryption scheme to MediaKeySystemMediaCapability dictionary. r=smaug

Add encryption scheme member to MediaKeySystemMediaCapability dict, but hide it
behind a pref as the behaviour involved is only proposed at this stage.

Differential Revision: https://phabricator.services.mozilla.com/D34301

--HG--
extra : moz-landing-system : lando

dom/webidl/MediaKeySystemAccess.webidl

@@ -19,6 +19,8 @@ enum MediaKeysRequirement {
 dictionary MediaKeySystemMediaCapability {
    DOMString contentType = "";
    DOMString robustness = "";
+   [Pref="media.eme.encrypted-media-encryption-scheme.enabled"]
+   DOMString? encryptionScheme = null;
 };
 
 dictionary MediaKeySystemConfiguration {