WebRTC: Remove build_with_chromium=false flag for GN

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_win_x64_gn_dbg.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "win_x64_gn_dbg",
@@ skipping 77 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gn.py",
       "--root=[SLAVE_BUILD]\\src",
       "gen",
       "//out/Debug_x64",
-      "--args=is_debug=true target_cpu=\"x64\" use_goma=true goma_dir=\"[BUILD]\\goma\" build_with_chromium=false"
+      "--args=is_debug=true target_cpu=\"x64\" use_goma=true goma_dir=\"[BUILD]\\goma\""
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\compile.py",
       "--target",
@@ skipping 19 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]