GitLab

- Converts IPC messages in midi_messages.h to mojo interfaces
  (in midi_service.mojom)
- Updates MidiHost and MidiMessageFilter to use mojo
- Changes MidiMessageFilter to do all work on the main task runner
- Renames MidiMessageFilter to MidiClientImpl

Bug: 582328
Change-Id: I1a62af005dc31118aad0081c30b6fbd1f63068a3
Reviewed-on: https://chromium-review.googlesource.com/c/1238815
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611747}