aura: Introduce EventInjector.

Introduce EventInjector, which can be used to inject events into aura.
In LOCAL mode, the events are directly injected on aura::WindowTreeHost.
In MUS mode, EventInjector injects the event at the window-server using
the WindowServerTest api.

BUG=721640

Review-Url: https://codereview.chromium.org/2869273010
Cr-Commit-Position: refs/heads/master@{#471806}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index f0d256a..1fa2766 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -62,6 +62,8 @@
     "env_input_state_controller.cc",
     "env_input_state_controller.h",
     "env_observer.h",
+    "event_injector.cc",
+    "event_injector.h",
     "input_state_lookup.cc",
     "input_state_lookup.h",
     "input_state_lookup_win.cc",