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

BUG=562675

Review URL: https://codereview.chromium.org/1479313002

Cr-Original-Commit-Position: refs/heads/master@{#362135}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b634f17a82d488df8851049c6ec2ad160b3547d3
diff --git a/tools/gn/header_checker.cc b/tools/gn/header_checker.cc
index b15b4fa..05b624c 100644
--- a/tools/gn/header_checker.cc
+++ b/tools/gn/header_checker.cc
@@ -163,9 +163,11 @@
         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;