libyuv: Disable signing for iOS

scripts/slave/recipes/libyuv/libyuv.expected/tryserver_libyuv_ios_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/cleanup_temp.py"
     ],
@@ skipping 132 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Release-iphoneos",
-      "--args=is_clang=true target_os=\"ios\" symbol_level=2 dcheck_always_on=true is_debug=false target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "--args=is_clang=true ios_enable_code_signing=false target_os=\"ios\" symbol_level=2 dcheck_always_on=true is_debug=false target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "gn"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
@@ skipping 59 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]