Add an output_dir override to GN.

tools/gn/tool.h

Index: tools/gn/tool.h
diff --git a/tools/gn/tool.h b/tools/gn/tool.h
index 9360ce66ef646381b07d7a904902349cedfcf58d..f6727ed6d16761ad58569e969def2ef92bcd4add 100644
--- a/tools/gn/tool.h
+++ b/tools/gn/tool.h
@@ -52,6 +52,14 @@ class Tool {
     default_output_extension_ = ext;
   }
 
+  const SubstitutionPattern& default_output_dir() const {
+    return default_output_dir_;
+  }
+  void set_default_output_dir(const SubstitutionPattern& dir) {
+    DCHECK(!complete_);
+    default_output_dir_ = dir;
+  }
+
   // Dependency file (if supported).
   const SubstitutionPattern& depfile() const {
     return depfile_;
@@ -186,6 +194,7 @@ class Tool {
  private:
   SubstitutionPattern command_;
   std::string default_output_extension_;
+  SubstitutionPattern default_output_dir_;
   SubstitutionPattern depfile_;
   DepsFormat depsformat_;
   PrecompiledHeaderType precompiled_header_type_;