Pick up ozone source when use_ozone = true.
Don't link X11 unless use_x11 = true.

BUG=angleproject:1297
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_gn_chromeos_rel

Change-Id: I94e6242189646c8a18b00436294bccdd4c5a8785
Reviewed-on: https://chromium-review.googlesource.com/352251Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>