EME: Update MediaKeyMessageEvent to match spec

MediaKeyMessageEventInit is now required for the constructor.

BUG=666869
TEST=W3C EME test mentioned in the bug no longer fails.

Review-Url: https://codereview.chromium.org/2514933002
Cr-Commit-Position: refs/heads/master@{#433323}

third_party/WebKit/Source/modules/encryptedmedia/MediaKeyMessageEvent.idl

@@ -32,7 +32,7 @@ enum MediaKeyMessageType {
 };
 
 [
-    Constructor(DOMString type, optional MediaKeyMessageEventInit eventInitDict)
+    Constructor(DOMString type, MediaKeyMessageEventInit eventInitDict)
 ] interface MediaKeyMessageEvent : Event {
     readonly attribute MediaKeyMessageType messageType;
     readonly attribute ArrayBuffer message;