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

tools/gn/item.cc

Index: tools/gn/item.cc
diff --git a/tools/gn/item.cc b/tools/gn/item.cc
index 4fb65d181f876e13c4016f89b8ed4c5e6de45b37..d698c087986aa7c589ffdaf422ef5e531c39808d 100644
--- a/tools/gn/item.cc
+++ b/tools/gn/item.cc
@@ -7,9 +7,13 @@
 #include "base/logging.h"
 #include "tools/gn/settings.h"
 
-Item::Item(const Settings* settings, const Label& label)
-    : settings_(settings), label_(label), defined_from_(nullptr) {
-}
+Item::Item(const Settings* settings,
+           const Label& label,
+           const std::set<uint32_t>& source_files_hashes)
+    : settings_(settings),
+      label_(label),
+      defined_from_(nullptr),
+      source_files_hashes_(source_files_hashes) {}
 
 Item::~Item() {
 }