Make clang_warning_flags work for standalone builds.

build/set_clang_warning_flags.gypi

+# Copyright (c) 2014 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 file is meant to be included to set clang-specific compiler flags.
+# To use this the following variable can be defined:
+#   clang_warning_flags:       list: Compiler flags to pass to clang.
+#   clang_warning_flags_unset: list: Compiler flags to not pass to clang.
+#
+# Only use this in third-party code. In chromium_code, fix your code to not
+# warn instead!
+#
+# Note that the gypi file is included in target_defaults, so it does not need
+# to be explicitly included.
+#
+# Warning flags set by this will be used on all platforms. If you want to set
+# warning flags on only some platforms, you have to do so manually.
+#
+# To use this, create a gyp target with the following form:
+# {
+#   'target_name': 'my_target',
+#   'variables': {
+#     'clang_warning_flags': ['-Wno-awesome-warning'],
+#     'clang_warning_flags_unset': ['-Wpreviously-set-flag'],
+#   }
+# }
+
+{
+  'variables': {
+    'clang_warning_flags_unset%': [],  # Provide a default value.
+  },
+  'conditions': [
+    ['clang==1', {
+      # This uses >@ instead of @< to also see clang_warning_flags set in
+      # targets directly, not just the clang_warning_flags in target_defaults.
+      'cflags': [ '>@(clang_warning_flags)' ],
+      'cflags!': [ '>@(clang_warning_flags_unset)' ],
+      'xcode_settings': {
+        'WARNING_CFLAGS': ['>@(clang_warning_flags)'],
+        'WARNING_CFLAGS!': ['>@(clang_warning_flags_unset)'],
+      },
+      'msvs_settings': {
+        'VCCLCompilerTool': {
+          'AdditionalOptions': [ '>@(clang_warning_flags)' ],
+          'AdditionalOptions!': [ '>@(clang_warning_flags_unset)' ],
+        },
+      },
+    }],
+    ['clang==0 and host_clang==1', {
+      'target_conditions': [
+        ['_toolset=="host"', {
+          'cflags': [ '>@(clang_warning_flags)' ],
+          'cflags!': [ '>@(clang_warning_flags_unset)' ],
+        }],
+      ],
+    }],
+  ],
+}