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

build/gyp_chromium.py

Index: build/gyp_chromium.py
diff --git a/build/gyp_chromium.py b/build/gyp_chromium.py
index 678dfd1047536133c0918ec340f844371f9891e2..2ba2a32f3e98a3b2521d638ba78e7e771f78a5db 100644
--- a/build/gyp_chromium.py
+++ b/build/gyp_chromium.py
@@ -284,6 +284,13 @@ def main():
     print 'Did you mean to use the `msvs-ninja` generator?'
     sys.exit(1)
 
+  # We explicitly don't support the native xcode gyp generator. Be nice and
+  # fail here, rather than generating broken projects.
+  if re.search(r'(^|,|\s)xcode($|,|\s)', os.environ.get('GYP_GENERATORS', '')):
+    print 'Error: xcode gyp generator not supported (check GYP_GENERATORS).'
+    print 'Did you mean to use the `xcode-ninja` generator?'
+    sys.exit(1)
+
   # If CHROMIUM_GYP_SYNTAX_CHECK is set to 1, it will invoke gyp with --check
   # to enfore syntax checking.
   syntax_check = os.environ.get('CHROMIUM_GYP_SYNTAX_CHECK')