Shadowing key is used to exclude non-enumerable keys when iterating
over the prototype chain. This CL skips adding shadowing key for
end prototype to improve for-in performance. It can improve the
performance of below microbench by ~50%.

Object.prototype.foo = function() {};
let obj = {
	a:0,
	b:1
};
let start = Date.now();
for (let i = 0; i<1e6; i++) {
	for (var j in obj) {}
}
console.log(Date.now() - start);

This CL also improves the score of JetStream2-tagcloud-SP case
by 8% on IA Chromebook.

Contributed by tao.pan@intel.com

Change-Id: I456082c08bf70f1f450ff54f657cdab26eb7bc2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1781113Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shiyu Zhang <shiyu.zhang@intel.com>
Cr-Commit-Position: refs/heads/master@{#63587}