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Current implementation monitors DBT_DEVICEARRIVAL mesage, but that
is not a right message to monitor device changes since actual device
list that OS holds will be updated a little later. Instead, monitor
DBT_DEVNODES_CHANGES that is delivered after the list being updated.

In Chrome, SystemMessageWindowWin invokes base::SystemMonitor's observer
with DEVTYPE_UNKNOWN for DBT_DEVNODES_CHANGED, and the SystemMonitor
provides a way to observe the event on the registered thread.
Using the monitor must be the right implementation.

BUG=472980

Review URL: https://codereview.chromium.org/1056333002

Cr-Commit-Position: refs/heads/master@{#324031}