Add outputs to GN's header checker.

tools/gn/action_values.cc

Index: tools/gn/action_values.cc
diff --git a/tools/gn/action_values.cc b/tools/gn/action_values.cc
index 80bea62a7da149ad4e83021ce282acd3f0e34426..2215167da366442b1db3a4afe885177ca543aedb 100644
--- a/tools/gn/action_values.cc
+++ b/tools/gn/action_values.cc
@@ -4,8 +4,26 @@
 
 #include "tools/gn/action_values.h"
 
+#include "tools/gn/substitution_writer.h"
+#include "tools/gn/target.h"
+
 ActionValues::ActionValues() {
 }
 
 ActionValues::~ActionValues() {
 }
+
+void ActionValues::GetOutputsAsSourceFiles(
+    const Target* target,
+    std::vector<SourceFile>* result) const {
+  if (target->output_type() == Target::COPY_FILES ||
+      target->output_type() == Target::ACTION_FOREACH) {
+    // Copy and foreach applies the outputs to the sources.
+    SubstitutionWriter::ApplyListToSources(
+        target->settings(), outputs_, target->sources(), result);
+  } else {
+    // Actions (and anything else that happens to specify an output) just use
+    // the output list with no substitution.
+    SubstitutionWriter::GetListAsSourceFiles(outputs_, result);
+  }
+}