Previously ViewportTest would rely on the scissor being set to the
size of the window by default, which is a fragile assumption as the
scissor is set to the size of the window only on the first makeCurrent.

BUG=angleproject:1105

Change-Id: I088565c7032fe817eb5c48a5351cc678fe42b376
Reviewed-on: https://chromium-review.googlesource.com/291310Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>