Add outputs to GN's header checker.

tools/gn/action_values.h

Index: tools/gn/action_values.h
diff --git a/tools/gn/action_values.h b/tools/gn/action_values.h
index fcb6acab24da0380dcd55888613f08feb623675c..3b546f29dd1b3ba70d4cfc45c1c04b4a3d5221e6 100644
--- a/tools/gn/action_values.h
+++ b/tools/gn/action_values.h
@@ -12,6 +12,8 @@
 #include "tools/gn/source_file.h"
 #include "tools/gn/substitution_list.h"
 
+class Target;
+
 // Holds the values (outputs, args, script name, etc.) for either an action or
 // an action_foreach target.
 class ActionValues {
@@ -32,6 +34,10 @@ class ActionValues {
   SubstitutionList& outputs() { return outputs_; }
   const SubstitutionList& outputs() const { return outputs_; }
 
+  // Expands the outputs() above to the final SourceFile list.
+  void GetOutputsAsSourceFiles(const Target* target,
+                               std::vector<SourceFile>* result) const;
+
   // Depfile generated by the script.
   const SubstitutionPattern& depfile() const { return depfile_; }
   bool has_depfile() const { return !depfile_.ranges().empty(); }