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Remove the stub version of DeviceDataManager that is created in the
browser and ash processes when running under mash. The stub version
never has any input-device information because the real
DeviceDataManager is in the mus process.

Create InputDeviceClient inside ash and chrome which connects to mus and
receives input-device updates via Mojo IPC.

The mus process starts InputDeviceServer to listen for observers and
update them when new input-device information is available.

BUG=601981

Review-Url: https://codereview.chromium.org/2058853002
Cr-Commit-Position: refs/heads/master@{#400746}