Default test window size to 16x16. 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/298835 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>