Convert GN visibility to be a list.

tools/gn/visibility.cc

Index: tools/gn/visibility.cc
diff --git a/tools/gn/visibility.cc b/tools/gn/visibility.cc
index c9dbb874897d5ab2f280c06842edb47b26f802e9..0dbad167cf0c6b38b607977176148b4b37df7da5 100644
--- a/tools/gn/visibility.cc
+++ b/tools/gn/visibility.cc
@@ -25,16 +25,10 @@ bool Visibility::Set(const SourceDir& current_dir,
                      Err* err) {
   patterns_.clear();
 
-  // Allow a single string to be passed in to make the common case (just one
-  // pattern) easier to specify.
-  if (value.type() == Value::STRING) {
-    patterns_.push_back(LabelPattern::GetPattern(current_dir, value, err));
-    return !err->has_error();
-  }
-
-  // If it's not a string, it should be a list of strings.
-  if (!value.VerifyTypeIs(Value::LIST, err))
+  if (!value.VerifyTypeIs(Value::LIST, err)) {
+    CHECK(err->has_error());
     return false;
+  }
 
   const std::vector<Value>& list = value.list_value();
   for (size_t i = 0; i < list.size(); i++) {