Enable debug layers by default whenever assertions are enabled. BUG=angleproject:2279 Change-Id: I7e7ef359fa3021aa10585653b2fec169f5d4a51f Reviewed-on: https://chromium-review.googlesource.com/794536 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>