GN: Use all targets to determine whether a file is generated (for `check`).

tools/gn/header_checker.cc

Index: tools/gn/header_checker.cc
diff --git a/tools/gn/header_checker.cc b/tools/gn/header_checker.cc
index b15b4fa16b2b88aaf91d57426adf8a225c6f9a0d..05b624cac2e77480b4b180e112ea071a6bbedf95 100644
--- a/tools/gn/header_checker.cc
+++ b/tools/gn/header_checker.cc
@@ -163,9 +163,11 @@ void HeaderChecker::RunCheckOverFiles(const FileMap& files, bool force_check) {
         type != SOURCE_M && type != SOURCE_MM && type != SOURCE_RC)
       continue;
 
-    // If any target marks it as generated, don't check it.
+    // If any target marks it as generated, don't check it. We have to check
+    // file_map_, which includes all known files; files only includes those
+    // being checked.
     bool is_generated = false;
-    for (const auto& vect_i : file.second)
+    for (const auto& vect_i : file_map_[file.first])
       is_generated |= vect_i.is_generated;
     if (is_generated)
       continue;