MidiManagerWinrt: Use midi::TaskService instead of MidiScheduler

To support dynamic instantiation mode even on this experimental
backend, use midi::TaskService instead of MidiScheduler.

Since this is the last consumer of MidiScheduler, this patch also
remove the MidiScheduler implementation from media/midi.

Bug: 672793
Change-Id: Ie50aa5ba403fb4c8a3ed28b18fb526ccad217d91
Reviewed-on: https://chromium-review.googlesource.com/839700
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529681}
8 files changed