gn format: penalty-based scheme for line breaking

tools/gn/format_test_data/046.gn

+# 80 ---------------------------------------------------------------------------
+if (true) {
+  # The JavaScript files required by main.html.
+  remoting_webapp_main_html_js_files =
+      # Include the core files first as it is required by the other files.
+      # Otherwise, Jscompile will complain.
+      remoting_webapp_js_core_files +
+      remoting_webapp_js_auth_client2host_files +
+      remoting_webapp_js_auth_google_files +
+      remoting_webapp_js_client_files +
+      remoting_webapp_js_gnubby_auth_files +
+      remoting_webapp_js_cast_extension_files +
+      remoting_webapp_js_host_files +
+      remoting_webapp_js_logging_files +
+      remoting_webapp_js_ui_files +
+      remoting_webapp_js_ui_host_control_files +
+      remoting_webapp_js_ui_host_display_files +
+      remoting_webapp_js_wcs_container_files
+      # Uncomment this line to include browser test files in the web app
+      # to expedite debugging or local development.
+      #+ remoting_webapp_js_browser_test_files
+}