gn: Check GN_EDITOR before EDITOR in environment variables.

tools/gn/command_args.cc

Index: tools/gn/command_args.cc
diff --git a/tools/gn/command_args.cc b/tools/gn/command_args.cc
index e2f4a9c09a7f11acab47339bbc0163ee60ec49d3..8098ea18f9e51f164ad6f4ab5ceb53c4981dd306 100644
--- a/tools/gn/command_args.cc
+++ b/tools/gn/command_args.cc
@@ -196,9 +196,10 @@ bool RunEditor(const base::FilePath& file_to_edit) {
 #else  // POSIX
 
 bool RunEditor(const base::FilePath& file_to_edit) {
-  // Prefer $VISUAL, then $EDITOR, then vi.
   const char* editor_ptr = getenv("VISUAL");
   if (!editor_ptr)
+    editor_ptr = getenv("GN_EDITOR");
+  if (!editor_ptr)
     editor_ptr = getenv("EDITOR");
   if (!editor_ptr)
     editor_ptr = "vi";