GitLab

Also, remove remaining RefPtr<MIDIPort>, the last user of one of the converters.

BUG=357163

Review URL: https://codereview.chromium.org/331493007

git-svn-id: svn://svn.chromium.org/blink/trunk@176063 bbb929c8-8fbe-4397-9dbb-9b2b20218538