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Previously the code queried TestScreen. This is problematic as
content_browsertests uses TestScreen, which *always* returns a display
id of 2000. When mus gets the event the event is then dropped (because
it doesn't know about 2000). This changes EventInjector to query
WindowTreeHost and has WindowTreeHostMus override GetDisplayId() to
return the real display id.

BUG=786453
TEST=none

Change-Id: I01b9ce9096a0a3e7d6ca9cf85706afb0213035a7
Reviewed-on: https://chromium-review.googlesource.com/809493Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521813}