math.js: Use %_TypedArrayGetLength to get length

test/mjsunit/regress/regress-615776.js

Index: test/mjsunit/regress/regress-615776.js
diff --git a/test/mjsunit/regress/regress-615776.js b/test/mjsunit/regress/regress-615776.js
new file mode 100644
index 0000000000000000000000000000000000000000..7e89b569c1f03eea8815c334f3cb4a5bf1bc34a0
--- /dev/null
+++ b/test/mjsunit/regress/regress-615776.js
@@ -0,0 +1,14 @@
+// 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.
+
+Object.defineProperty(Int32Array.prototype.__proto__, 'length', {
+  get: function() { throw new Error('Custom length property'); }
+});
+
+var a = Math.random();
+
+// This tests MathRandomRaw.
+var v0 = new Set();
+var v1 = new Object();
+v0.add(v1);