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Currently the code assumes that a TYPE_DEFAULT MesssageLoop will be backed by
CFRunLoop, which is required to receive the CoreMIDI callbacks. In the future,
that assumption may not be true, so explicitly request a TYPE_UI (CFRunLoop)
pump.

Bug: 753544
Change-Id: I5464e1299d8cb99d6fbf37d8b342417487117744
Reviewed-on: https://chromium-review.googlesource.com/682834
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504448}