Flip all but ios-simulator builders to GN.

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

 {
   "owners": [
     "smut"
   ],
   "comments": [
     "Cronet tests for 32- and 64-bit iOS 9.0 simulators.",
     "Tests run on iPhone 5s (64-bit) and iPad Retina (32-bit)."
   ],
   "xcode version": "7.3",
   "GYP_DEFINES": [
     "OS=ios",
     "chromium_ios_signing=0",
     "component=static_library",
     "enable_websockets=false",
     "disable_brotli_filter=true",
     "disable_file_support=true",
     "disable_ftp_support=true",
     "gomadir=$(goma_dir)",
     "target_subarch=both",
     "use_goma=1",
     "use_platform_icu_alternatives=true"
   ],
-  "mb_type": "gyp",
+  "gn_args": [
+    "additional_target_cpus=[\"x86\"]",
+    "component=static_library",
+    "disable_brotli_filter=true",
+    "disable_file_support=true",
+    "disable_ftp_support=true",
+    "enable_websockets=false",
+    "goma_dir=\"$(goma_dir)\"",
+    "target_cpu=\"x64\"",
+    "target_os=\"ios\"",
+    "use_platform_icu_alternatives=true"
+  ],
+  "mb_type": "gn",
   "compiler": "ninja",
+  "additional_compiler_targets": [ "gn_all" ],
   "configuration": "Debug",
   "sdk": "iphonesimulator9.3",
   "tests": [
     {
       "app": "cronet_test",
       "device type": "iPhone 5s",
       "os": "9.3"
     },
     {
       "app": "cronet_test",
       "device type": "iPad Retina",
       "os": "9.3"
     }
   ]
 }