[debugger] Don't leak holes from generator arguments.

test/mjsunit/regress/regress-5164.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Flags: --expose-debug-as debug
+
+var failure = null;
+var args;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != debug.Debug.DebugEvent.Break) return;
+  try {
+    args = exec_state.frame(0).evaluate('arguments').value();
+  } catch (e) {
+    failure = e;
+  }
+}
+
+debug.Debug.setListener(listener);
+
+function* gen(a, b) {
+  debugger;
+  yield a;
+  yield b;
+}
+
+var foo = gen(1, 2);
+
+foo.next()
+assertEquals(2, args.length);
+assertEquals(undefined, args[0]);
+assertEquals(undefined, args[1]);
+
+foo.next()
+assertEquals(2, args.length);
+assertEquals(undefined, args[0]);
+assertEquals(undefined, args[1]);
+
+foo.next()
+assertEquals(2, args.length);
+assertEquals(undefined, args[0]);
+assertEquals(undefined, args[1]);
+
+assertNull(failure);