[lacros/chrome] Enforce use_gtk=false if chromeos_is_browser_only

This prevents the developer from accidentally enabling use_gtk=true
on the lacros/chrome build, and diverge from the ChromeOS environment.

BUG=None
R=jamescook@chromium.org

Change-Id: Idc204d85b970d47fe96dc21dec55c1ac091b8484
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343488
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Auto-Submit: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#796146}

chromeos/lacros/BUILD.gn

@@ -3,9 +3,11 @@
 # found in the LICENSE file.
 
 import("//build/config/chromeos/ui_mode.gni")
+import("//build/config/linux/gtk/gtk.gni")
 
 # Code lives in the lacros-chrome browser only, not ash-chrome.
 assert(chromeos_is_browser_only)
+assert(!use_gtk)
 
 # Reset sources_assignment_filter for the BUILD.gn file to prevent
 # regression during the migration of Chromium away from the feature.