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

Reland with a speculative fix for ViewportTest

BUG=angleproject:1105

Change-Id: I83e89881de5b6f58cfb3e14cbe324fd9547f2836
Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/288533Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>