This fixes a name conflict with the XWindow headers on Linux.
Only becomes an issue when using Optional inside GL-level libANGLE.

BUG=angleproject:1123

Change-Id: I7f0eb278a8b6eb6fecd864e556428678cb90dcea
Reviewed-on: https://chromium-review.googlesource.com/298892Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>