Remove custom V8 bindings for MIDI

We can remove the custom bindings using [SetReference].
[SetReference] generates code that is semantically the same as the custom bindings.

Review URL: https://codereview.chromium.org/140783007

git-svn-id: svn://svn.chromium.org/blink/trunk@165286 bbb929c8-8fbe-4397-9dbb-9b2b20218538