These builders set use_x11=1 but don't have GLX headers. Avoid trying
to compile ANGLE's GL, GLX and X11 sources on Chrome OS at all.

BUG=503173,angleproject:892

Change-Id: I7f5862bb83b33f6ce302244fa5668295789a3a41
Reviewed-on: https://chromium-review.googlesource.com/281778Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>