Templatize the return type of requestMIDIAccess()

From external contributor RetroModular.

Fixes issue #119
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=66246030

Author: blickly

contrib/externs/w3c_midi.js

@@ -24,7 +24,7 @@
 
 /**
  * @param {!MIDIOptions=} opt_options
- * @return {!Promise}
+ * @return {!Promise.<!MIDIAccess>}
  */
 navigator.requestMIDIAccess = function(opt_options) {};