Test for generated files being detected during `gn check`.

tools/gn/header_checker_unittest.cc

Index: tools/gn/header_checker_unittest.cc
diff --git a/tools/gn/header_checker_unittest.cc b/tools/gn/header_checker_unittest.cc
index 19d5e4054fd4f850d0f6672180166c57b7ca8abb..f719247079b63ce58ca4283a4ce64437f68e1a6d 100644
--- a/tools/gn/header_checker_unittest.cc
+++ b/tools/gn/header_checker_unittest.cc
@@ -288,3 +288,31 @@ TEST_F(HeaderCheckerTest, CheckIncludeAllowCircular) {
   EXPECT_TRUE(checker->CheckInclude(&b_, input_file, a_public, range, &err));
   EXPECT_FALSE(err.has_error());
 }
+
+TEST_F(HeaderCheckerTest, SkipGeneratedFilesFromOtherTargets) {
+  Err err;
+
+  Target action(setup_.settings(), Label(SourceDir("//a"), "generate_file"));
+  action.set_output_type(Target::ACTION);
+  action.SetToolchain(setup_.toolchain(), &err);
+  action.visibility().SetPublic();
+  action.action_values().outputs() =
+      SubstitutionList::MakeForTest("//out/Debug/gen/a/a.h");
+
+  Target target(setup_.settings(), Label(SourceDir("//a"), "use_file"));
+  target.set_output_type(Target::SOURCE_SET);
+  target.SetToolchain(setup_.toolchain(), &err);
+  target.visibility().SetPublic();
+  target.sources().push_back(SourceFile("//out/Debug/gen/a/a.h"));
+
+  std::vector<const Target*> all_targets;
+  all_targets.push_back(&action);
+  all_targets.push_back(&target);
+  std::vector<const Target*> to_check;
+  to_check.push_back(&target);
+  bool force_check = false;
+  std::vector<Err> errs;
+  scoped_refptr<HeaderChecker> checker(
+      new HeaderChecker(setup_.build_settings(), all_targets));
+  EXPECT_TRUE(checker->Run(to_check, force_check, &errs));
+}