GitLab

This CL:
- Makes PointerDeviceObserver use ui::InputDeviceEventObserver on X11 (it is
  already used on Ozone)
- Makes X11HotplugEventObserver notify ui::DeviceDataManager when a mouse /
  touchpad is plugged in / unplugged
- Removes DeviceHierarchyObserver

BUG=440503
TEST=Manual

Review URL: https://codereview.chromium.org/946173002

Cr-Commit-Position: refs/heads/master@{#318997}