Fix initialization of udev in MidiManagerAlsa.
Fix initialization of udev in MidiManagerAlsa.

New devices would not be detected on Linux. Worse,
things would get very confused if you disconnected
one device and connected another.

This has been broken for four months, with
crrev.com/17cd1b10ded3a5e730bbcb39c57a6d2f8e16a51e

BUG=625123

Review-Url: https://codereview.chromium.org/2126023004
Cr-Commit-Position: refs/heads/master@{#404092}
(cherry picked from commit e74c2a9687c28f2718d71ae6ad9b6153119361c7)

TBR=agoode@chromium.org

Review URL: https://codereview.chromium.org/2135903002 .

Cr-Commit-Position: 03d981dee550804c6934fd1a1d9cfa3c172affd3@{#606}
Cr-Branched-From: e9621a186760eb36d74098d9e33e4dd91c427b34@{#394939}

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