Decouple MIDIAccess initialization from MIDIAccess class.

public/web/WebMIDIClient.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 20 matching lines @@
 #ifndef WebMIDIClient_h
 #define WebMIDIClient_h
 
 namespace blink {
 class WebMIDIPermissionRequest;
 
 class WebMIDIClient {
 public:
     virtual ~WebMIDIClient() { }
 
-    // Request a permission to use system exclusive messages. Called when MIDIOptions.sysex is true.
-    virtual void requestSysexPermission(const WebMIDIPermissionRequest&) = 0;
-    // Cancel the request since the requesting frame may be moving to a new page.
-    virtual void cancelSysexPermissionRequest(const WebMIDIPermissionRequest&) = 0;
+    // Request a permission to use system exclusive messages. Called when
+    // MIDIOptions.sysex is true.
+    // Once the request is done, any WebMIDIPermissionRequest instances equal to
+    // |request| must not be accessed after that.
+    virtual void requestSysexPermission(const WebMIDIPermissionRequest& /* request */) = 0;
+
+    // Cancel the request since the requesting frame may be moving to
+    // a new page.
+    // Once canceled, any WebMIDIPermissionRequest instances equal to
+    // |request| must not be accessed after that.
+    virtual void cancelSysexPermissionRequest(const WebMIDIPermissionRequest& /* request */) = 0;
 };
 
 } // namespace blink
 
 #endif // WebMIDIClient_h