Use ninja as the default GYP generator on FreeBSD.

While here, consistently use a single 'if' statement to set this
environment variable.

BUG=

Review URL: https://codereview.chromium.org/176843005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254856 0039d316-1c4b-4281-b951-d872f2087c98

build/gyp_chromium

@@ -524,9 +524,8 @@ if __name__ == '__main__':
   # . -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 sys.platform.startswith('win') and not os.environ.get('GYP_GENERATORS'):
+  if sys.platform.startswith(('linux', 'win', 'freebsd')) 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', []):