[debugger] be more lenient about argument type in %DebugGetProperty.

test/mjsunit/regress/regress-crbug-629996.js

Index: test/mjsunit/regress/regress-crbug-629996.js
diff --git a/test/mjsunit/regress/regress-crbug-629996.js b/test/mjsunit/regress/regress-crbug-629996.js
new file mode 100644
index 0000000000000000000000000000000000000000..025a86ee72ed2bc576c9acdd409bb4bae08a08dc
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-629996.js
@@ -0,0 +1,9 @@
+// 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 mirror = debug.MakeMirror(new Proxy({}, {}));
+// As long as we have no special mirror for proxies, we use an object mirror.
+assertEquals("object", mirror.type());