Remove compiler field from iOS bot configs, ensure gn_args match configuration

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

 {
   "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": "8.0",
   "gn_args": [
     "additional_target_cpus=[\"x86\"]",
     "disable_brotli_filter=true",
     "disable_file_support=true",
     "disable_ftp_support=true",
     "enable_websockets=false",
     "goma_dir=\"$(goma_dir)\"",
     "is_component_build=false",
     "is_cronet_build=true",
+    "is_debug=true",
     "target_cpu=\"x64\"",
     "target_os=\"ios\"",
     "use_platform_icu_alternatives=true"
   ],
-  "compiler": "ninja",
   "configuration": "Debug",
   "sdk": "iphonesimulator10.0",
   "tests": [
     {
       "app": "cronet_test",
       "device type": "iPhone 5s",
       "os": "9.3"
     },
     {
       "app": "cronet_test",
       "device type": "iPad Retina",
       "os": "9.3"
     }
   ]
 }