Flip remaining builders to GN.

ios/build/bots/chromium.mac/ios-simulator.json

 {
   "owners": [
     "dpranke",
     "smut"
   ],
   "comments": [
     "Tests for 32- and 64-bit iOS 9.0 simulators.",
     "Tests run on iPhone 5s (64-bit) and iPad Retina (32-bit).",
-    "Build is performed with gyp+ninja."
+    "Build is performed with gn+ninja."
   ],
   "xcode version": "7.0",
   "GYP_DEFINES": [
     "OS=ios",
     "chromium_ios_signing=0",
     "component=static_library",
     "use_goma=1",
     "gomadir=$(goma_dir)",
     "target_subarch=both"
   ],
   "gn_args": [
     "is_debug=true",
     "target_os=\"ios\"",
     "ios_enable_code_signing=false",
     "is_component_build=false",
     "use_goma=true",
     "goma_dir=\"$(goma_dir)\"",
     "target_cpu=\"x64\"",
     "additional_target_cpus=[\"x86\"]"
   ],
-  "mb_type": "gyp",
+  "mb_type": "gn",
   "compiler": "ninja",
   "configuration": "Debug",
   "sdk": "iphonesimulator9.0",
   "tests": [
     {
       "include": "common_tests.json",
       "device type": "iPhone 5s",
       "os": "9.0"
     },
     {
       "include": "common_tests.json",
       "device type": "iPad Retina",
       "os": "9.0"
     }
   ]
 }