Let gyp create ninja files by default on linux.

build/gyp_chromium

 #!/usr/bin/env python
 
 # Copyright (c) 2012 The Chromium 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 Chromium that adds some support for how GYP
 # is invoked by Chromium beyond what can be done in the gclient hooks.
 
 import glob
@@ skipping 117 matching lines @@
   # currently exist.  The check for circular dependencies is currently
   # bypassed on other platforms, but is left enabled on the Mac, where a
   # violation of the rule causes Xcode to misbehave badly.
   # TODO(mark): Find and kill remaining circular dependencies, and remove this
   # option.  http://crbug.com/35878.
   # TODO(tc): Fix circular dependencies in ChromiumOS then add linux2 to the
   # list.
   if sys.platform not in ('darwin',):
     args.append('--no-circular-check')
 
+  # Default to ninja on linux, but only if no generator has explicitly been set.
+  # . -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') and not os.environ.get('GYP_GENERATORS'):
+    os.environ['GYP_GENERATORS'] = 'ninja'
+
   # 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')
   if syntax_check and int(syntax_check):
     args.append('--check')
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Off we go...
   sys.exit(gyp.main(args))