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The class is extracted from EventRouter of Files.app.
It is going to manage device states and dispatch the device events.

BUG=360946, 396258
TEST=add unit tests

Review URL: https://codereview.chromium.org/472603004

Cr-Commit-Position: refs/heads/master@{#291384}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291384 0039d316-1c4b-4281-b951-d872f2087c98