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Since platform_event_observers_ is unused, this change removes it and
related code.

Bug: 855308
Change-Id: I717d8d3676dd4788e2dc27cc2f8d757fac591648
Reviewed-on: https://chromium-review.googlesource.com/1137151
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576384}