Only one EGL_NONE is needed to signal the end of the list.

BUG=angleproject:892

Change-Id: I7de349bef9d674344ccd1b2c4801d8b42c801299
Reviewed-on: https://chromium-review.googlesource.com/271157Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>