This fixes a crash on Linux whenever we try to create a test without
specifying a default width and height. Did not seem to have a problem
on Windows.

TEST=angle_end2end_tests (Windows/Linux)
BUG=angleproject:1123

Change-Id: I1afcea9e833208d8a67d363678b9736c2a222066
Reviewed-on: https://chromium-review.googlesource.com/298835Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>