GN: do not set default GN args

infra/bots/recipe_modules/flavor/gn_flavor.py

 # Copyright 2016 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.
 
 import default_flavor
 
 """GN flavor utils, used for building Skia with GN."""
 class GNFlavorUtils(default_flavor.DefaultFlavorUtils):
   def supported(self):
     extra_config = self.m.vars.builder_cfg.get('extra_config', '')
 
     return any([
       extra_config == 'GN',
       extra_config == 'Fast',
       extra_config.startswith('SK')
     ])
 
   def _run(self, title, cmd):
     path = self.m.vars.default_env['PATH']
     self.m.vars.default_env = {'PATH': path}
     self.m.run(self.m.step, title, cmd=cmd, cwd=self.m.vars.skia_dir)
 
   def compile(self, unused_target, **kwargs):
     """Build Skia with GN."""
     compiler      = self.m.vars.builder_cfg.get('compiler',      '')
     configuration = self.m.vars.builder_cfg.get('configuration', '')
     extra_config  = self.m.vars.builder_cfg.get('extra_config',  '')
     os            = self.m.vars.builder_cfg.get('os',            '')
 
-    cc, cxx = 'cc', 'c++'
+    cc, cxx = None, None
     extra_cflags = []
     extra_ldflags = []
 
     if compiler == 'Clang' and os == 'Ubuntu':
       cc  = self.m.vars.slave_dir.join('clang_linux', 'bin', 'clang')
       cxx = self.m.vars.slave_dir.join('clang_linux', 'bin', 'clang++')
       extra_ldflags.append('-fuse-ld=lld')
     elif compiler == 'Clang':
       cc, cxx = 'clang', 'clang++'
     elif compiler == 'GCC':
       cc, cxx = 'gcc', 'g++'
 
     if extra_config == 'Fast':
       extra_cflags.extend(['-march=native', '-fomit-frame-pointer', '-O3'])
     if extra_config.startswith('SK'):
       extra_cflags.append('-D' + extra_config)
 
-    quote = lambda x: '"%s"' % x
-    gn_args = ' '.join('%s=%s' % (k,v) for (k,v) in sorted({
-        'cc': quote(cc),
-        'cxx': quote(cxx),
-        'extra_cflags': quote(' '.join(extra_cflags)),
-        'extra_ldflags': quote(' '.join(extra_ldflags)),
-        'is_debug': 'true' if configuration == 'Debug' else 'false',
-    }.iteritems()))
+    args = {}
+
+    if configuration != 'Debug':
+      args['is_debug'] = 'false'
+
+    for (k,v) in {
+      'cc':  cc,
+      'cxx': cxx,
+      'extra_cflags':  ' '.join(extra_cflags),
+      'extra_ldflags': ' '.join(extra_ldflags),
+    }.iteritems():
+      if v:
+        args[k] = '"%s"' % v
+
+    gn_args = ' '.join('%s=%s' % (k,v) for (k,v) in sorted(args.iteritems()))
 
     self._run('fetch-gn', [self.m.vars.skia_dir.join('bin', 'fetch-gn')])
     self._run('gn gen', ['gn', 'gen', self.out_dir, '--args=' + gn_args])
     self._run('ninja', ['ninja', '-C', self.out_dir])