This avoids a flickering of the window at the start of every test.

BUG=angleproject:1105

Change-Id: Ib277ad5cfef97e00d27408f8feaa8ccba9a5e032
Reviewed-on: https://chromium-review.googlesource.com/287640Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>