[WebMIDI] Remove content/renderer/media/midi

- Moves functionality in content::RendererWebMIDIAccessorImpl to
  blink::MIDIAccessor, and functionality in
  content::MidiSessionClientImpl to blink::MIDIDispatcher.
- Removes blink::WebMIDIAccessor and blink::WebMIDIAccessorClient.
- Replaces MockMIDIWebAccessor with a JS implementation of
  midi::mojom::MidiSessionProvider and updates tests to use it.

Bug: 582328
Change-Id: I48fccfc20d85915450a0b738ad471edba18d3a8b
Reviewed-on: https://chromium-review.googlesource.com/c/1289955
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616699}
53 files changed