libyuv: Switch bots over to GN by default.

scripts/slave/recipes/webrtc/libfuzzer.expected/full_tryserver_webrtc_linux_libfuzzer_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/external/webrtc'}, {'deps_file': 'DEPS', 'managed': True, 'name': 'webrtc-limited', 'url': 'https://chrome-internal.googlesource.com/chrome/deps/webrtc-limited'}]",
       "--patch_root",
       "src",
@@ skipping 118 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Release",
-      "--args=ffmpeg_branding=\"Chrome\" rtc_use_h264=true use_libfuzzer=true is_asan=true is_debug=false target_cpu=\"x64\""
+      "--args=ffmpeg_branding=\"Chrome\" rtc_use_h264=true use_libfuzzer=true is_asan=true symbol_level=2 is_debug=false target_cpu=\"x64\""
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gn.py",
       "--root=[SLAVE_BUILD]/src",
@@ skipping 62 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]