This allows the test code to assume that the framebuffer size is equal
to the window size.

BUG=angleproject:1050

Change-Id: Ia4928d7a1d6cddb1ae330846bb44576db6a37533
Reviewed-on: https://chromium-review.googlesource.com/286253Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>