Add an output_dir override to GN.

tools/gn/substitution_type.h

 // Copyright 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.
 
 #ifndef TOOLS_GN_SUBSTITUTION_TYPE_H_
 #define TOOLS_GN_SUBSTITUTION_TYPE_H_
 
 #include <vector>
 
 class Err;
@@ skipping 38 matching lines @@
   SUBSTITUTION_CFLAGS_OBJC,  // {{cflags_objc}}
   SUBSTITUTION_CFLAGS_OBJCC,  // {{cflags_objcc}}
   SUBSTITUTION_DEFINES,  // {{defines}}
   SUBSTITUTION_INCLUDE_DIRS,  // {{include_dirs}}
 
   // Valid for linker tools.
   SUBSTITUTION_LINKER_INPUTS,  // {{inputs}}
   SUBSTITUTION_LINKER_INPUTS_NEWLINE,  // {{inputs_newline}}
   SUBSTITUTION_LDFLAGS,  // {{ldflags}}
   SUBSTITUTION_LIBS,  // {{libs}}
+  SUBSTITUTION_OUTPUT_DIR,  // {{output_dir}}
   SUBSTITUTION_OUTPUT_EXTENSION,  // {{output_extension}}
   SUBSTITUTION_SOLIBS,  // {{solibs}}
 
   // Valid for bundle_data targets.
   SUBSTITUTION_BUNDLE_ROOT_DIR,  // {{bundle_root_dir}}
   SUBSTITUTION_BUNDLE_RESOURCES_DIR,  // {{bundle_resources_dir}}
   SUBSTITUTION_BUNDLE_EXECUTABLE_DIR,  // {{bundle_executable_dir}}
   SUBSTITUTION_BUNDLE_PLUGINS_DIR,  // {{bundle_plugins_dir}}
 
   // Used only for the args of actions.
@@ skipping 51 matching lines @@
 bool IsValidCompileXCassetsSubstitution(SubstitutionType type);
 
 // Like the "IsValid..." version above but checks a list of types and sets a
 // an error blaming the given source if the test fails.
 bool EnsureValidSourcesSubstitutions(
     const std::vector<SubstitutionType>& types,
     const ParseNode* origin,
     Err* err);
 
 #endif  // TOOLS_GN_SUBSTITUTION_TYPE_H_