Add optional public header checking to GN build

tools/gn/setup.h

Index: tools/gn/setup.h
diff --git a/tools/gn/setup.h b/tools/gn/setup.h
index 0a5c3427730419300d0747587c69f3143c838ad2..6b85da3cd1a3d59f05c99ed50cb27202f462302a 100644
--- a/tools/gn/setup.h
+++ b/tools/gn/setup.h
@@ -48,6 +48,12 @@ class CommonSetup {
     check_for_unused_overrides_ = s;
   }
 
+  // After a successful run, setting this will additionally cause the public
+  // headers to be checked. Defaults to false.
+  void set_check_public_headers(bool s) {
+    check_public_headers_ = s;
+  }
+
   BuildSettings& build_settings() { return build_settings_; }
   Builder* builder() { return builder_.get(); }
   LoaderImpl* loader() { return loader_.get(); }
@@ -67,6 +73,7 @@ class CommonSetup {
 
   bool check_for_bad_items_;
   bool check_for_unused_overrides_;
+  bool check_public_headers_;
 
  private:
   CommonSetup& operator=(const CommonSetup& other);  // Disallow.