Allow GN configs to have sub-configs.

tools/gn/config_values.h

Index: tools/gn/config_values.h
diff --git a/tools/gn/config_values.h b/tools/gn/config_values.h
index 5452826cd708812c37126fca4d5dda6eb4a06773..d053e4a3c692d7900975e165f62ad42aa8a57ad5 100644
--- a/tools/gn/config_values.h
+++ b/tools/gn/config_values.h
@@ -19,6 +19,9 @@ class ConfigValues {
   ConfigValues();
   ~ConfigValues();
 
+  // Appends the values from the given config to this one.
+  void AppendValues(const ConfigValues& append);
+
 #define STRING_VALUES_ACCESSOR(name) \
     const std::vector<std::string>& name() const { return name##_; } \
     std::vector<std::string>& name() { return name##_; }
@@ -36,6 +39,7 @@ class ConfigValues {
   STRING_VALUES_ACCESSOR(ldflags)
   DIR_VALUES_ACCESSOR   (lib_dirs)
   STRING_VALUES_ACCESSOR(libs)
+  // If you add a new one, be sure to update AppendValues().
 
 #undef STRING_VALUES_ACCESSOR
 #undef DIR_VALUES_ACCESSOR
@@ -67,11 +71,10 @@ class ConfigValues {
   std::vector<std::string> ldflags_;
   std::vector<SourceDir>   lib_dirs_;
   std::vector<std::string> libs_;
+  // If you add a new one, be sure to update AppendValues().
 
   std::string precompiled_header_;
   SourceFile precompiled_source_;
-
-  DISALLOW_COPY_AND_ASSIGN(ConfigValues);
 };
 
 #endif  // TOOLS_GN_CONFIG_VALUES_H_