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This code will be shared by HidServiceLinux and UsbServiceLinux so it
needs to be in the shared //device/core component.

Sharing the service introduces an issue with destruction order that is
resolved by adding a WillDestroyMonitorMessageLoop() method to its
Observer interface.

BUG=422562

Review-Url: https://codereview.chromium.org/2014933002
Cr-Commit-Position: refs/heads/master@{#396318}