For some reason while running the end2end tests, when the application
gets an NSAppKitDefinedEventType with subtype
NSApplicationActivateEventType (and of course window = 0), a null
dereference happens. The crash seems timing dependent, since the root
cause was not found and this is test code, add an ugly suppression.

BUG=angleproject:1570

Change-Id: I3af2188ddae8ca3c0a4458e15a8af01e66215e3f
Reviewed-on: https://chromium-review.googlesource.com/403049Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>