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To handle device connection status, MidiManager and MidiMessageFilter
hold all devices as a list. These lists are modified by event
notifications that are delivered asynchronously.

They do not remove disconnected devices, but just mark as disconnected.
This is useful when JavaScript has a reference to a device
that got disconnected. The reference can be availalbe if the device
gets connected again.

BUG=422333
TEST=media_unittests

Review URL: https://codereview.chromium.org/662233002

Cr-Commit-Position: refs/heads/master@{#317000}