Implement tracking of BUILD.gn files used to define target, toolchain or

tools/gn/item.h

Index: tools/gn/item.h
diff --git a/tools/gn/item.h b/tools/gn/item.h
index 3ec482af1df7b11445f07eee1dcb139f48c28ae9..2c2233f3bd2e5a52fe909d7d2b88b8f830d09f0b 100644
--- a/tools/gn/item.h
+++ b/tools/gn/item.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "tools/gn/input_file.h"
 #include "tools/gn/label.h"
 #include "tools/gn/visibility.h"
 
@@ -21,7 +22,9 @@ class Toolchain;
 // graph.
 class Item {
  public:
-  Item(const Settings* settings, const Label& label);
+  Item(const Settings* settings,
+       const Label& label,
+       const InputFileSet& input_files);
   virtual ~Item();
 
   const Settings* settings() const { return settings_; }
@@ -55,12 +58,15 @@ class Item {
   // returns false on failure.
   virtual bool OnResolved(Err* err);
 
+  const InputFileSet& input_files() const { return input_files_; }
+
  private:
   const Settings* settings_;
   Label label_;
   const ParseNode* defined_from_;
 
   Visibility visibility_;
+  InputFileSet input_files_;
 };
 
 #endif  // TOOLS_GN_ITEM_H_