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We rely on the implicit copy constructors in a large number of places
in the code. However, the existance of ui::Event::Clone() makes people
think they have to use that method to copy events -- they don't, if
they have an event with a known type.

Write out the copy constructors to make that clear.

Bug: none
Test: existing events_unittests, other existing UI unittests
Change-Id: I18cd94bdc1cdec9d91c2987da3dd28356edd8568
Reviewed-on: https://chromium-review.googlesource.com/830674Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524488}