Add W3C encrypted-media tests

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

+(function(){
+    if( navigator.userAgent.toLowerCase().indexOf('edge') === -1
+            && navigator.userAgent.toLowerCase().indexOf('chrome') > -1){
+
+        if ( ( /chrome\/([0-9]*)\./.exec( navigator.userAgent.toLowerCase() )[1] | 0 ) < 54 ) {
+
+            // Work around https://bugs.chromium.org/p/chromium/issues/detail?id=622956
+            // Chrome does not fire the empty keystatuschange event when a session is closed
+            var _mediaKeySessionClose = MediaKeySession.prototype.close;
+            var _mediaKeySessionKeyStatusesGetter = Object.getOwnPropertyDescriptor( MediaKeySession.prototype, 'keyStatuses' ).get;
+            var _emptyMediaKeyStatusMap = { size: 0,
+                                            has:    function() { return false; },
+                                            get:    function() { return undefined; },
+                                            entries:function() { return []; },          // this may not be correct, I think it should be some iterator thing
+                                            keys:   function() { return []; },
+                                            values: function() { return []; },
+                                            forEach:function() { return; } };
+
+            MediaKeySession.prototype.close = function close()
+            {
+                this.__closed = true;
+
+                setTimeout( function() {
+                        this.dispatchEvent( new Event( 'keystatuseschange' ) );
+                }.bind( this ), 0 );
+
+                return _mediaKeySessionClose.call( this );
+            };
+
+            Object.defineProperty( MediaKeySession.prototype, 'keyStatuses', { get: function() {
+
+                return this.__closed ? _emptyMediaKeyStatusMap : _mediaKeySessionKeyStatusesGetter.call( this );
+
+            } } );
+        }
+    }
+}());