[debugger] correctly find function context.

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index dc4c0bd2c6f659775d734db0f6187eb80ebf2c16..7549d205182731b525d69674db7bed8e0e3cdd31 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -79,6 +79,15 @@ Context* Context::declaration_context() {
   return current;
 }
 
+Context* Context::closure_context() {
+  Context* current = this;
+  while (!current->IsFunctionContext() && !current->IsScriptContext() &&
+         !current->IsNativeContext()) {
+    current = current->previous();
+    DCHECK(current->closure() == closure());
+  }
+  return current;
+}
 
 JSObject* Context::extension_object() {
   DCHECK(IsNativeContext() || IsFunctionContext() || IsBlockContext());