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Since AppKit coordinates are flipped, NSWindowDelegate may not send
move changes when the top-left corner of the window moves relative to
the screen. Similarly, it will send a move when resizing a window
vertically via the bottom edge, which other platforms do not class as a
move.

To fix, track the top-left window corner relative to the top-left of the
screen and ensure the toolkit is notified when it changes.

OnWidgetMove() had no test coverage, so add a cross-platform widget test.
The test exposes a discrepancy on Windows only, which is likely a bug to
investigate in a follow-up.

Bug: 862217, 864938
Change-Id: I1fb0a6c2b463ea7cbd823fbe4d65545991564d50
Reviewed-on: https://chromium-review.googlesource.com/1141555Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576255}