Enhance GN's "desc" command to support configs.

tools/gn/config_values.h

 // 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.
 
 #ifndef TOOLS_GN_CONFIG_VALUES_H_
 #define TOOLS_GN_CONFIG_VALUES_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 12 matching lines @@
   // 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##_; }
 #define DIR_VALUES_ACCESSOR(name) \
     const std::vector<SourceDir>& name() const { return name##_; } \
     std::vector<SourceDir>& name() { return name##_; }
 
-  // ==================================================================
-  // IMPORTANT: If you add a new one, be sure to update AppendValues().
-  // ==================================================================
+  // =================================================================
+  // IMPORTANT: If you add a new one, be sure to update AppendValues()
+  //            and command_desc.cc.
+  // =================================================================
   STRING_VALUES_ACCESSOR(arflags)
   STRING_VALUES_ACCESSOR(asmflags)
   STRING_VALUES_ACCESSOR(cflags)
   STRING_VALUES_ACCESSOR(cflags_c)
   STRING_VALUES_ACCESSOR(cflags_cc)
   STRING_VALUES_ACCESSOR(cflags_objc)
   STRING_VALUES_ACCESSOR(cflags_objcc)
   STRING_VALUES_ACCESSOR(defines)
   DIR_VALUES_ACCESSOR   (include_dirs)
   STRING_VALUES_ACCESSOR(ldflags)
   DIR_VALUES_ACCESSOR   (lib_dirs)
-  // ==================================================================
-  // IMPORTANT: If you add a new one, be sure to update AppendValues().
-  // ==================================================================
+  // =================================================================
+  // IMPORTANT: If you add a new one, be sure to update AppendValues()
+  //            and command_desc.cc.
+  // =================================================================
 
 #undef STRING_VALUES_ACCESSOR
 #undef DIR_VALUES_ACCESSOR
 
   const std::vector<LibFile>& libs() const { return libs_; }
   std::vector<LibFile>& libs() { return libs_; }
 
   bool has_precompiled_headers() const {
     return !precompiled_header_.empty() || !precompiled_source_.is_null();
   }
@@ skipping 23 matching lines @@
   std::vector<std::string> ldflags_;
   std::vector<SourceDir>   lib_dirs_;
   std::vector<LibFile>     libs_;
   // If you add a new one, be sure to update AppendValues().
 
   std::string precompiled_header_;
   SourceFile precompiled_source_;
 };
 
 #endif  // TOOLS_GN_CONFIG_VALUES_H_