[debugger] correctly find function context.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index d01ddafccc651221cf88bd5898dc553d7d31586b..5035a4ba03eac0475dd84dde18f688b9f4f60548 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -450,6 +450,9 @@ class Context: public FixedArray {
   Context* declaration_context();
   bool is_declaration_context();
 
+  // Get the next closure's context on the context chain.
+  Context* closure_context();
+
   // Returns a JSGlobalProxy object or null.
   JSObject* global_proxy();
   void set_global_proxy(JSObject* global);