chromeos: creates TestEventInjector for tests to inject events

This interface is *only* exposed in tests. It's to be used for tests that
want to generate events and ensure they are processed.

My plan is remove the existing interface
(services/ui/public/interfaces/event_injector.mojom) soonish.

BUG=852439
TEST=covered by tests

Change-Id: I2df1e5d428564f78b7280087a4f5453ce76b55b7
Reviewed-on: https://chromium-review.googlesource.com/1144472
Commit-Queue: Scott Violet <[email protected]>
Reviewed-by: Tom Sepez <[email protected]>
Reviewed-by: Sadrul Chowdhury <[email protected]>
Cr-Commit-Position: refs/heads/master@{#577724}
27 files changed