This will allow us to use EGLAttrib and pass pointers as Display
init parameters if necessary.

BUG=angleproject:2042

Change-Id: Ib0f85d71bc9d3dff2db9453012f40303f3c3bd7c
Reviewed-on: https://chromium-review.googlesource.com/513518Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>