[es6] Match ArrayBuffer constructor behavior of Firefox.

We choose to deliberately violate the ES2015 specification and implement
the ArrayBuffer constructor in a way that matches Firefox and Safari
instead.

BUG=v8:4592
LOG=n

Review URL: https://codereview.chromium.org/1497163002

Cr-Commit-Position: refs/heads/master@{#32597}

src/builtins.cc

@@ -1781,7 +1781,7 @@ BUILTIN(ArrayBufferConstructor_ConstructStub) {
   isolate->set_context(args.target()->context());
   Handle<Object> number_length;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, number_length,
-                                     Object::ToNumber(length));
+                                     Object::ToInteger(isolate, length));
   size_t byte_length;
   if (!TryNumberToSize(isolate, *number_length, &byte_length)) {
     THROW_NEW_ERROR_RETURN_FAILURE(