Add a flag to BUILDCONFIG for updating some global configs.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 137 matching lines @@
   if (current_os == "chromeos") {
     # Allows the target toolchain to be injected as arguments. This is needed
     # to support the CrOS build system which supports per-build-configuration
     # toolchains.
     cros_use_custom_toolchain = false
   }
 
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
 
+# This flag indicates if the build is using the new optimization and symbol
+# level configs, or the old ones.
+#
+# This is part of a multi-repo landing. When all required deps (WebKit and
+# ffmpeg) have been updated to handle this flag, we can change the optimization
+# and symbol level configs and set this flag to true. Then delete the users of
+# the flag, wait for all deps rolls, then remove the flag.
+#
+# In the old scheme, this file sets the appropriate optimization or symbol
+# level config on targets depending on what the current build setup is. This
+# requires that targets know what the default is, and if they want to change it,
+# they have to figure out which one to remove depending on the state of the
+# current build. It also means that symbol_level and the sanitizer flags need
+# to be global.
+#
+# In the new scheme, this file sets a "default_symbols" and
+# "default_optimization" configs, which then expands to the right thing
+# depending on the current build. This means less information has to be global,
+# and there's only one config to remove that's the same in all cases for
+# targets that want to override it. It also means that the associated flags can
+# be non-global.
+using_new_global_compiler_configs = false
+
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
 #
 # We set these various is_FOO booleans for convenience in writing OS-based
 # conditions.
 #
 # - is_android, is_chromeos, is_ios, and is_win should be obvious.
 # - is_mac is set only for desktop Mac. It is not set on iOS.
 # - is_posix is true for mac and any Unix-like system (basically everything
@@ skipping 450 matching lines @@
     forward_variables_from(invoker, "*")
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     if (!defined(deps)) {
       deps = []
     }
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }