Set use_goma=1 and gomadir=path in GYP_DEFINES on master.

build/scripts/slave/compile.py

 #!/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.
 
 """A tool to build chrome, executed by buildbot.
 
   When this is run, the current directory (cwd) should be the outer build
   directory (e.g., chrome-release/build/).
 
@@ skipping 1125 matching lines @@
                            help='build mode (dev or official) controlling '
                                 'environment variables set during build')
   option_parser.add_option('--build-tool', default=None,
                            help='specify build tool (ib, vs, xcode)')
   option_parser.add_option('--build-args', action='append', default=[],
                            help='arguments to pass to the build tool')
   option_parser.add_option('--compiler', default=None,
                            help='specify alternative compiler (e.g. clang)')
   if chromium_utils.IsWindows():
     # Windows only.
-    option_parser.add_option('', '--no-ib', action='store_true', default=False,
+    option_parser.add_option('--no-ib', action='store_true', default=False,
                              help='use Visual Studio instead of IncrediBuild')
-    option_parser.add_option('', '--msvs_version',
+    option_parser.add_option('--msvs_version',
                              help='VisualStudio version to use')
   # For linux to arm cross compile.
-  option_parser.add_option('', '--crosstool', default=None,
+  option_parser.add_option('--crosstool', default=None,
                            help='optional path to crosstool toolset')
-  option_parser.add_option('', '--llvm-tsan', action='store_true',
+  option_parser.add_option('--llvm-tsan', action='store_true',
                            default=False,
                            help='build with LLVM\'s ThreadSanitizer')
   if chromium_utils.IsMac():
     # Mac only.
-    option_parser.add_option('', '--xcode-target', default=None,
+    option_parser.add_option('--xcode-target', default=None,
                              help='Target from the xcodeproj file')
-  option_parser.add_option('', '--goma-dir',
+  option_parser.add_option('--goma-dir',
                            default=os.path.join(BUILD_DIR, 'goma'),
                            help='specify goma directory')
   option_parser.add_option('--verbose', action='store_true')
 
   options, args = option_parser.parse_args()
 
   if not options.src_dir:
     options.src_dir = 'src'
   options.src_dir = os.path.abspath(options.src_dir)
 
@@ skipping 64 matching lines @@
   options.target_output_dir = get_target_build_dir(options.build_tool,
       options.src_dir, options.target, 'iphoneos' in args)
   options.clobber = (options.clobber or
       landmines_triggered(options.target_output_dir))
 
   return main(options, args)
 
 
 if '__main__' == __name__:
   sys.exit(real_main())