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Make EventRewriterController create the keyboard rewriter itself.
Simplify EventRewriterController, add rewriters immediately.

Add ash::mojom::EventRewriterController for use by Chrome/OOBE.
(one central interface seems better than one for each rewriter)

This should allow Mash to support this rewriter in the future.
(for now, Ash doesn't get Window Service events sent to Chrome)

Bug: 647781
Test: No Hangouts Meet keyboard driven OOBE regressions; see Issue 251272
Change-Id: I7b731298d612b1d93dab825b12a9d36431d1b711
Reviewed-on: https://chromium-review.googlesource.com/1033424Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554801}