[parsing] Fix maybe-assigned for loop variables.

src/ast/scopes.h

Index: src/ast/scopes.h
diff --git a/src/ast/scopes.h b/src/ast/scopes.h
index a1661d122618e1f6293cb664e3646afda3b6f708..ea37890c5e4875406996b0a5916141fc2459fc82 100644
--- a/src/ast/scopes.h
+++ b/src/ast/scopes.h
@@ -181,6 +181,7 @@ class V8_EXPORT_PRIVATE Scope : public NON_EXPORTED_BASE(ZoneObject) {
                             bool* sloppy_mode_block_scope_function_redefinition,
                             bool* ok);
 
+  // The return value is meaningful only if FLAG_preparser_scope_analysis is on.
   Variable* DeclareVariableName(const AstRawString* name, VariableMode mode);
 
   // Declarations list.