Don't hard code "chromeos" to zero.
Fix incorrect assumptions that X11 would always be used on linux.
Introduce "angle_use_drm" as the Chrome OS alternative to "angle_use_x11."
Define USE_OZONE for Chrome OS.
Allow overriding pkg-config.

BUG=angleproject:1297

Change-Id: Ie85b5c37f39170e0d11f8fc1bb70c175db6ee334
Reviewed-on: https://chromium-review.googlesource.com/323610Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>