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This patch adds an additional function to the UsbService::Observer
interface that should be used to perform cleanup operations when a USB
device is disconnected. This is separate from the OnDeviceRemoved
function so that implementations of that function, which may depend on
other subsystems' knowledge of a connected device, can run before any
cleanup actions have taken place.

A browsertest for the chrome.usb.getUserSelectedDevices is included
which demonstrates the need for this 2-phase event handling.

BUG=452298

Review URL: https://codereview.chromium.org/891853002

Cr-Commit-Position: refs/heads/master@{#314973}