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Converting MIDIDispatcher also allows us to use the HeapMojo wrappers
to manage the mojo connections.

BUG=chromium:1049056

Change-Id: I1870796a2c99036537f09be71206eeafe59dda11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417468Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#808268}