Add W3C encrypted-media tests

third_party/WebKit/LayoutTests/imported/wpt/encrypted-media/polyfill/firefox-polyfill.js

+(function(){
+    if(navigator.userAgent.toLowerCase().indexOf('firefox') > -1){
+
+        // Work around https://bugzilla.mozilla.org/show_bug.cgi?id=1282142
+        // Firefox does not correctly reject the Clear Key session types it does not support
+        var _requestMediaKeySystemAccess = navigator.requestMediaKeySystemAccess.bind( navigator );
+
+        navigator.requestMediaKeySystemAccess = function( keysystem, configurations )
+        {
+            if ( keysystem !== 'org.w3.clearkey' ) return _requestMediaKeySystemAccess( keysystem, configurations );
+
+            var supported_configurations = configurations.filter( function( c ) {
+
+                return !c.sessionTypes || ( c.sessionTypes.length === 1 && c.sessionTypes[ 0 ] === 'temporary' );
+
+            } );
+
+            if ( supported_configurations.length === 0 ) return Promise.reject( new DOMException( 'None of the requested configurations were supported.' ) );
+
+            return _requestMediaKeySystemAccess( keysystem, supported_configurations );
+        }
+    }
+}());