The Mac AMD GPU FYI bots were crashing on end2end tests exit because the
NSView was trying to deallocated an NSTrackingArea that had been
previously deallocated.

BUG=angleproject:891

Change-Id: I933e437254cad65f18cb0518a401b8b8e9dc59a0
Reviewed-on: https://chromium-review.googlesource.com/303302Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>