Add a flag to BUILDCONFIG for updating some global configs.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 6bc28eccf710cf6e6b2f40609d1a79b7ef67837b..99b54d26ed1e4e55c55b7117449e395c04b20b7a 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -155,6 +155,29 @@ declare_args() {
   # 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
 # =============================================================================