Let Runtime_GrowArrayElements accept non-Smi numbers as |key|.

test/mjsunit/regress/regress-crbug-485410.js

+// Copyright 2015 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: --allow-natives-syntax
+
+var doubles = new Float64Array(1);
+function ToHeapNumber(i) {
+  doubles[0] = i;
+  return doubles[0];
+}
+for (var i = 0; i < 3; i++) ToHeapNumber(i);
+%OptimizeFunctionOnNextCall(ToHeapNumber);
+assertFalse(%IsSmi(ToHeapNumber(1)));
+
+function Fail(a, i, v) {
+  a[i] = v;
+}
+
+for (var i = 0; i < 3; i++) Fail(new Array(1), 1, i);
+%OptimizeFunctionOnNextCall(Fail);
+// 1050 > JSObject::kMaxGap, causing stub failure and runtime call.
+Fail(new Array(1), ToHeapNumber(1050), 3);