ios: Default GYP_GENERATORS to ninja, and error out if GYP_GENERATORS is xcode

build/gyp_environment.py

Index: build/gyp_environment.py
diff --git a/build/gyp_environment.py b/build/gyp_environment.py
index 199023adf12d65907497b17d5c762df6d36a3026..87e3ea8680f0778afa37fb4143b9143751441a85 100644
--- a/build/gyp_environment.py
+++ b/build/gyp_environment.py
@@ -24,12 +24,9 @@ def SetEnvironment():
   # . -f / --format has precedence over the env var, no need to check for it
   # . set the env var only if it hasn't been set yet
   # . chromium.gyp_env has been applied to os.environ at this point already
-  if sys.platform.startswith(('linux', 'win', 'freebsd')) and \
+  if sys.platform.startswith(('linux', 'win', 'freebsd', 'darwin')) and \
       not os.environ.get('GYP_GENERATORS'):
     os.environ['GYP_GENERATORS'] = 'ninja'
-  elif sys.platform == 'darwin' and not os.environ.get('GYP_GENERATORS') and \
-      not 'OS=ios' in os.environ.get('GYP_DEFINES', []):
-    os.environ['GYP_GENERATORS'] = 'ninja'
 
   vs_toolchain.SetEnvironmentAndGetRuntimeDllDirs()
   mac_toolchain.SetToolchainEnvironment()