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

tools/gn/ninja_group_target_writer_unittest.cc

Index: tools/gn/ninja_group_target_writer_unittest.cc
diff --git a/tools/gn/ninja_group_target_writer_unittest.cc b/tools/gn/ninja_group_target_writer_unittest.cc
index 9feb4bd6059efae6e381f80956946b5ae8e25b36..e2636a6709a795c7e4eda3d09999b163084b7cee 100644
--- a/tools/gn/ninja_group_target_writer_unittest.cc
+++ b/tools/gn/ninja_group_target_writer_unittest.cc
@@ -11,23 +11,23 @@ TEST(NinjaGroupTargetWriter, Run) {
   Err err;
   TestWithScope setup;
 
-  Target target(setup.settings(), Label(SourceDir("//foo/"), "bar"));
+  Target target(setup.settings(), Label(SourceDir("//foo/"), "bar"), {});
   target.set_output_type(Target::GROUP);
   target.visibility().SetPublic();
 
-  Target dep(setup.settings(), Label(SourceDir("//foo/"), "dep"));
+  Target dep(setup.settings(), Label(SourceDir("//foo/"), "dep"), {});
   dep.set_output_type(Target::ACTION);
   dep.visibility().SetPublic();
   dep.SetToolchain(setup.toolchain());
   ASSERT_TRUE(dep.OnResolved(&err));
 
-  Target dep2(setup.settings(), Label(SourceDir("//foo/"), "dep2"));
+  Target dep2(setup.settings(), Label(SourceDir("//foo/"), "dep2"), {});
   dep2.set_output_type(Target::ACTION);
   dep2.visibility().SetPublic();
   dep2.SetToolchain(setup.toolchain());
   ASSERT_TRUE(dep2.OnResolved(&err));
 
-  Target datadep(setup.settings(), Label(SourceDir("//foo/"), "datadep"));
+  Target datadep(setup.settings(), Label(SourceDir("//foo/"), "datadep"), {});
   datadep.set_output_type(Target::ACTION);
   datadep.visibility().SetPublic();
   datadep.SetToolchain(setup.toolchain());