MB: Flip iOS bots to GN by default

webrtc/build/ios/client.webrtc/iOS64_Simulator_Debug.json

 {
   "owners": [
     "kjellander"
   ],
   "comments": [
     "Tests for 32-bit iOS 9.0 simulators.",
     "GYP_DEFINES and gn_args in this file are only used during runhooks; ",
     "webrtc/build/mb_config.pyl decides the generated projects that are compiled."
   ],
   "xcode version": "7.0",
   "GYP_DEFINES": [
     "use_goma=1",
     "gomadir=$(goma_dir)",
     "chromium_ios_signing=0",
     "component=static_library",
     "target_arch=x64"
   ],
   "gn_args": [
     "is_debug=true",
     "target_os=\"ios\"",
     "ios_enable_code_signing=false",
     "is_component_build=false",
     "use_goma=true",
     "goma_dir=\"$(goma_dir)\"",
     "target_cpu=\"x64\""
   ],
-  "mb_type": "gyp",
+  "mb_type": "gn",
   "compiler": "ninja",
+  "additional_compile_targets": [ "all" ],
   "configuration": "Debug",
   "sdk": "iphonesimulator9.0",
   "tests": [
     {
       "include": "common_tests.json",
       "device type": "iPhone 5s",
       "os": "9.0"
     }
   ]
 }