Improve GN public header file checking

tools/gn/visibility.cc

Index: tools/gn/visibility.cc
diff --git a/tools/gn/visibility.cc b/tools/gn/visibility.cc
index c03ef561bc51c3db033f5b22e7fc3b7cb63f05a2..41bfc63f412ef9a9734768be90bbd20071b41c79 100644
--- a/tools/gn/visibility.cc
+++ b/tools/gn/visibility.cc
@@ -78,11 +78,13 @@ bool Visibility::Set(const SourceDir& current_dir,
 }
 
 void Visibility::SetPublic() {
+  patterns_.clear();
   patterns_.push_back(
       VisPattern(VisPattern::RECURSIVE_DIRECTORY, SourceDir(), std::string()));
 }
 
 void Visibility::SetPrivate(const SourceDir& current_dir) {
+  patterns_.clear();
   patterns_.push_back(
       VisPattern(VisPattern::DIRECTORY, current_dir, std::string()));
 }