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- Changes a DCHECK to a CHECK to crash if a move loop starts when one is already
in progress.
- Adds a CHECK to ensure the event monitor's handler is running on the main thread.
- Adds crash keys in CocoaWindowMoveLoop's destructor to capture its address and
a stack trace, to test the hypothesis that crashes may be occurring on objects
that have already been freed.

Bug: 876493
Change-Id: Ic36703f08138daff88e46f02e7af3de61acedc4e
Reviewed-on: https://chromium-review.googlesource.com/1205012
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589243}