gyp_pdfium should use ninja by default.

build/gyp_pdfium

 #!/usr/bin/env python
 #
 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script is wrapper for PDFium that adds some support for how GYP
 # is invoked by PDFium beyond what can be done in the gclient hooks.
 
 import os
@@ skipping 10 matching lines @@
 def run_gyp(args):
   rc = gyp.main(args)
   if rc != 0:
     print 'Error running GYP'
     sys.exit(rc)
 
 
 def main():
   args = sys.argv[1:]
   args.append(os.path.join(script_dir, 'all.gyp'))
-  
+
   args.append('-I')
   args.append(os.path.join(pdfium_root, 'build', 'standalone.gypi'))
-  
+
   args.extend(['-D', 'gyp_output_dir=out'])
 
   # Set the GYP DEPTH variable to the root of the PDFium project.
   args.append('--depth=' + os.path.relpath(pdfium_root))
 
+  # GYP does not default to ninja, but PDFium should.
+  if not os.environ.get('GYP_GENERATORS', ''):
+    print 'GYP_GENERATORS is not set, defaulting to ninja'
+    os.environ['GYP_GENERATORS'] = 'ninja'
+
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   run_gyp(args)
 
 
 if __name__ == '__main__':
-  sys.exit(main())
+  sys.exit(main())