Rework iOS recipes to use latest MB approach

scripts/slave/recipes/ios/try.expected/gn.json

 [
   {
     "cmd": [
       "[BUILD]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "host and tools info",
     "~followup_annotations": [
@@ skipping 113 matching lines @@
       "[SLAVE_BUILD]/src/ios/build/bots/tryserver.fake/ios.json",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "read build config",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"GYP_DEFINES\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"compiler\": \"ninja\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"configuration\": \"Debug\", @@@",
+      "@@@STEP_LOG_LINE@json.output@  \"gn_args\": \"target_os=\\\"ios\\\" ios_enable_code_signing=false\", @@@",
+      "@@@STEP_LOG_LINE@json.output@  \"mb_type\": \"gn\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"sdk\": \"iphonesimulator8.0\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tests\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"app\": \"fake tests\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"device type\": \"fake device\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"os\": \"8.1\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  ], @@@",
-      "@@@STEP_LOG_LINE@json.output@  \"use_mb\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"xcode version\": \"fake xcode version\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "echo",
       "-e",
-      "{\n  \"GYP_DEFINES\": {\n    \"OS\": \"ios\", \n    \"component\": \"static_library\"\n  }, \n  \"additional_compile_targets\": [\n    \"All\"\n  ], \n  \"compiler\": \"ninja\", \n  \"configuration\": \"Debug\", \n  \"env\": {}, \n  \"sdk\": \"iphonesimulator8.0\", \n  \"tests\": [\n    {\n      \"app\": \"fake tests\", \n      \"device type\": \"fake device\", \n      \"os\": \"8.1\"\n    }\n  ], \n  \"triggered bots\": {}, \n  \"use_mb\": true, \n  \"xcode version\": \"fake xcode version\"\n}"
+      "{\n  \"GYP_DEFINES\": {}, \n  \"additional_compile_targets\": [\n    \"All\"\n  ], \n  \"compiler\": \"ninja\", \n  \"configuration\": \"Debug\", \n  \"env\": {}, \n  \"gn_args\": \"target_os=\\\"ios\\\" ios_enable_code_signing=false\", \n  \"mb_type\": \"gn\", \n  \"sdk\": \"iphonesimulator8.0\", \n  \"tests\": [\n    {\n      \"app\": \"fake tests\", \n      \"device type\": \"fake device\", \n      \"os\": \"8.1\"\n    }\n  ], \n  \"triggered bots\": {}, \n  \"xcode version\": \"fake xcode version\"\n}"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "finalize build config"
   },
   {
     "cmd": [
       "[BUILD]/scripts/slave/ios/find_xcode.py",
       "--json-file",
       "/path/to/tmp/json",
       "--version",
       "fake xcode version"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "find xcode",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
+      "GN_ARGS": "target_os=\"ios\" ios_enable_code_signing=false",
       "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_CROSSCOMPILE": "1",
-      "GYP_DEFINES": "component=static_library OS=ios",
       "GYP_GENERATORS": "ninja",
-      "LANDMINES_VERBOSE": "1"
+      "LANDMINES_VERBOSE": "1",
+      "MB_TYPE": "gn"
     },
     "name": "gclient runhooks (with patch)",
     "~followup_annotations": [
-      "@@@STEP_TEXT@<br />GYP_DEFINES:<br />component=static_library<br />OS=ios@@@"
+      "@@@STEP_TEXT@<br />gn args:<br />target_os=\"ios\" ios_enable_code_signing=false@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/tools/mb/mb.py",
       "gen",
       "-m",
       "tryserver.fake",
@@ skipping 83 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@This test was skipped because it was not affected.@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]