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

tools/gn/target.cc

Index: tools/gn/target.cc
diff --git a/tools/gn/target.cc b/tools/gn/target.cc
index 759d7bff14493e59663830c67f2259562eb28f11..5061539bc43df7f3a52ee08554cfe597e99f8b2f 100644
--- a/tools/gn/target.cc
+++ b/tools/gn/target.cc
@@ -275,8 +275,10 @@ Dependencies
   future, do not rely on this behavior.
 )";
 
-Target::Target(const Settings* settings, const Label& label)
-    : Item(settings, label),
+Target::Target(const Settings* settings,
+               const Label& label,
+               const std::set<uint32_t>& source_files_hashes)
+    : Item(settings, label, source_files_hashes),
       output_type_(UNKNOWN),
       output_prefix_override_(false),
       output_extension_set_(false),
@@ -284,8 +286,7 @@ Target::Target(const Settings* settings, const Label& label)
       check_includes_(true),
       complete_static_lib_(false),
       testonly_(false),
-      toolchain_(nullptr) {
-}
+      toolchain_(nullptr) {}
 
 Target::~Target() {
 }