This use after free was triggered when a window was made key and
destroyed, then another window was made key. dedResignKey was called on
the former's WindowDelegate which had already been freed, causing a
crash.

BUG=angleproject:891

Change-Id: If47e88419f9a407a6a77c5cd9853b7b4860dc698
Reviewed-on: https://chromium-review.googlesource.com/305817
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>