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This also moves ShowSystemMenuAtPoint() from gfx (why was it there?) to views.

BUG=none
TEST=Hit alt-space; system menu should appear below the tabstrip.

Change-Id: Iba2ca9fc54746f253ef7d61ed5b190d61de47f5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1049063Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670348}