When BROWSE_EXTENSIONS_DIR is set alongside GSTACK_CHROMIUM_PATH pointing
at a baked-extension build (GBrowser / GStack Browser), the headless launch()
path was unconditionally adding --disable-extensions-except / --load-extension.
This causes the same ServiceWorkerState::SetWorkerId DCHECK crash that
launchHeaded() already guards against via isCustomChromium().
Mirror the existing guard: skip --load-extension flags when isCustomChromium()
returns true; always push the off-screen window geometry args.
