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The spec[1] states that XRCoordinateSystem inherits EventTarget.
This patch implements the inheritance.

[1]: https://immersive-web.github.io/webxr/#xrcoordinatesystem-interface

Bug: None
Change-Id: I509ff04c8bbe76a499655fa48a726ac10f444716
Reviewed-on: https://chromium-review.googlesource.com/1189503
Commit-Queue: Byoungkwon Ko <codeimpl@gmail.com>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592632}