Fix off-by-one in Array.concat's max index check

The maximum valid index is strictly smaller than the maximum valid length.

BUG=chromium:516592
LOG=y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1278703003

Cr-Commit-Position: refs/heads/master@{#30040}

src/runtime/runtime-array.cc

@@ -133,7 +133,7 @@ class ArrayConcatVisitor {
   ~ArrayConcatVisitor() { clear_storage(); }
 
   void visit(uint32_t i, Handle<Object> elm) {
-    if (i > JSObject::kMaxElementCount - index_offset_) {
+    if (i >= JSObject::kMaxElementCount - index_offset_) {
       set_exceeds_array_limit(true);
       return;
     }

test/mjsunit/regress/regress-581.js

@@ -36,7 +36,6 @@ assertThrows(function() { a.concat(a); }, RangeError);
 
 var b = [];
 b[pow31 - 3] = 32;
-b[pow31 - 2] = "out_of_bounds";
 var ab = a.concat(b);
 assertEquals(2 * pow31 - 1, ab.length);
 assertEquals(31, ab[pow31]);

test/mjsunit/regress/regress-crbug-516592.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.
+
+var i = Math.pow(2, 31);
+var a = [];
+a[i] = 31;
+var b = [];
+b[i - 2] = 33;
+try {
+  // This is supposed to throw a RangeError.
+  var c = a.concat(b);
+  // If it didn't, ObservableSetLength will detect the problem.
+  Object.observe(c, function() {});
+  c.length = 1;
+} catch(e) {
+  assertTrue(e instanceof RangeError);
+}