Minor cleanups to MIDIClientProxy.

When I was looking and trying to read this API I noticed that some of
its includes and forward declarations were not really necessary, so
while I'm here, I removed them.

BUG=None
TEST=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/109633002

git-svn-id: svn://svn.chromium.org/blink/trunk@163383 bbb929c8-8fbe-4397-9dbb-9b2b20218538