Support -Goutput_dir=blahblah in GN-GYP hybrid mode

build/gyp_chromium

Index: build/gyp_chromium
diff --git a/build/gyp_chromium b/build/gyp_chromium
index f8fa93664642b73b23e673a31e8225c235e0dc1d..4998aaa062014f28fef49eae98f13ee2d027c203 100755
--- a/build/gyp_chromium
+++ b/build/gyp_chromium
@@ -127,6 +127,23 @@ def GetGypVarsForGN(supplemental_files):
 
   return dict(supp_items + env_items + cmdline_items)
 
+def GetOutputDirectory():
+  """Returns the output directory that GYP will use."""
+  # GYP generator flags from the command line. We can't use optparse since we
+  # want to ignore all arguments other than "-G".
+  needle = '-Goutput_dir='
+  cmdline_input_items = []
+  for item in sys.argv[1:]:
+    if item.startswith(needle):
+      return item[len(needle):]
+
+  env_items = shlex.split(os.environ.get('GYP_GENERATOR_FLAGS', ''))
+  needle = 'output_dir='
+  for item in env_items:
+    if item.startswith(needle):
+      return item[len(needle):]
+
+  return "out"
 
 def GetArgsStringForGN(supplemental_files):
   """Returns the args to pass to GN.
@@ -224,6 +241,10 @@ def GetArgsStringForGN(supplemental_files):
 
   # Set the GYP flag so BUILD files know they're being invoked in GYP mode.
   gn_args += ' is_gyp=true'
+
+  gyp_outdir = GetOutputDirectory()
+  gn_args += ' gyp_output_dir=\"%s\"' % gyp_outdir
+
   return gn_args.strip()
 
 
@@ -284,7 +305,8 @@ def RunGN(supplemental_includes):
   # to know they're being run under GYP.
   args = [gnpath, 'gyp', '-q',
           '--root=' + chrome_src,
-          '--args=' + GetArgsStringForGN(supplemental_includes)]
+          '--args=' + GetArgsStringForGN(supplemental_includes),
+          '--output=//' + GetOutputDirectory() + '/gn_build/']
   return subprocess.call(args) == 0
 
 
@@ -397,6 +419,8 @@ if __name__ == '__main__':
   args.extend(
       ['-I' + i for i in additional_include_files(supplemental_includes, args)])
 
+  args.extend(['-D', 'gyp_output_dir=' + GetOutputDirectory()])
+
   print 'Updating projects from gyp files...'
   sys.stdout.flush()