[elements] cleaning up string wrapper elements kind and adding tests

drive-by-fix: unify template parameters

BUG=chromium:586068
LOG=n

Review URL: https://codereview.chromium.org/1857163002

Cr-Commit-Position: refs/heads/master@{#35302}

test/mjsunit/string-wrapper.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.
+
+var limit = 10000;
+
+function testStringWrapper(string) {
+  assertEquals('a', string[0]);
+  assertEquals('b', string[1]);
+  assertEquals('c', string[2]);
+}
+
+(function testFastStringWrapperGrow() {
+  var string = new String("abc");
+  for (var i = 0; i < limit; i += 2) {
+    string[i] = {};
+  }
+  testStringWrapper(string);
+
+  for (var i = limit; i > 0; i -= 2) {
+    delete string[i];
+  }
+  testStringWrapper(string);
+})();
+
+(function testSlowStringWrapperGrow() {
+  var string = new String("abc");
+  // Force Slow String Wrapper Elements Kind
+  string[limit] = limit;
+  for (var i = 0; i < limit; i += 2) {
+    string[i] = {};
+  }
+  testStringWrapper(string);
+  assertEquals(limit, string[limit]);
+
+  for (var i = limit; i > 0; i -= 2) {
+    delete string[i];
+  }
+  testStringWrapper(string);
+  assertEquals(undefined, string[limit]);
+})();