gn: add Blink GC plugin options

tools/clang/scripts/blink_gc_plugin_flags.py

 #!/usr/bin/env python
 # Copyright 2015 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 returns the flags that should be passed to clang.
 
+# TODO(sof): the script can be removed when/once gyp support is retired;
+# unused with gn.
+
 import os
 import sys
 
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
 SRC_DIR = os.path.join(THIS_DIR, '..', '..', '..')
 CLANG_LIB_PATH = os.path.normpath(os.path.join(
     SRC_DIR, 'third_party', 'llvm-build', 'Release+Asserts', 'lib'))
 
 FLAGS = '-Xclang -add-plugin -Xclang blink-gc-plugin'
 PREFIX= ' -Xclang -plugin-arg-blink-gc-plugin -Xclang '
 
 for arg in sys.argv[1:]:
   if arg == 'dump-graph=1':
     FLAGS += PREFIX + 'dump-graph'
   elif arg == 'warn-unneeded-finalizer=1':
     FLAGS += PREFIX + 'warn-unneeded-finalizer'
   elif arg.startswith('custom_clang_lib_path='):
     CLANG_LIB_PATH = arg[len('custom_clang_lib_path='):]
   elif arg == 'target_os=ios':
     sys.stderr.write('error: blink is unsupported on iOS\n')
     sys.exit(1)
 
 if not sys.platform in ['win32', 'cygwin']:
   LIBSUFFIX = 'dylib' if sys.platform == 'darwin' else 'so'
   FLAGS = ('-Xclang -load -Xclang "%s/libBlinkGCPlugin.%s" ' + FLAGS) % \
            (CLANG_LIB_PATH, LIBSUFFIX)
 
 print FLAGS