MidiManagerUsb should not trust indices provided by renderer.

      MidiManagerUsb::DispatchSendMidiData takes |port_index| parameter. As it is
provided by a renderer possibly under the control of an attacker, we must
validate the given index before using it.

BUG=456516

Review URL: https://codereview.chromium.org/907793002

Cr-Commit-Position: refs/heads/master@{#315303}