Web MIDI: add unit tests for event notification of MidiManagerMac

Web MIDI: add unit tests for event notification of MidiManagerMac

Add a unit test to check if MidiManagerMac correctly notify clients.
Core MIDI requires that the thread calling MIDIClientCreate() should
run CFRunLoop. MidiManagerMac uses base::Thread to call
MIDIClientCreate() on an assumption that it uses CFRunLoop() inside.
To make the assumption confirmed, implement a test for MidiManagerMac.
The test creates a MIDIDestination and checks if AddOutputPort()
is called responded to the MIDIDestination.

BUG=422333
TEST=media_unittests

Review URL: https://codereview.chromium.org/666693002

Cr-Commit-Position: refs/heads/master@{#317247}