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This makes the injector use the same interface as the devices
themselves. It also allows us to remove the PostTask after ui::Event
construction from EventFactoryEvdev, because all dispatches already
start in their own task (posted by ForwardingDeviceEventDispatcher).

BUG=449710
TEST=boot link_freon & move mouse

Review URL: https://codereview.chromium.org/882503002

Cr-Commit-Position: refs/heads/master@{#313878}