On CGL, the default framebuffer will have a name different than 0 and
without this change it wouldn't get special-cased as on the other
platforms. It is assumed that the default framebuffer will get
initialized directly by the driver or other parts of the code.

BUG=angleproject:891

Change-Id: Ifbe4ada58f27ad9ddb5b43697c234cb17e7504f0
Reviewed-on: https://chromium-review.googlesource.com/290147Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>