There was a security issue from allowing WindowProxy to have its prototype mutated in https://bugs.chromium.org/p/chromium/issues/detail?id=399951 . This has since been resolved by making `window`, `location`, and `Object.prototype.__proto__` immutable. This change was fixed in https://bugs.chromium.org/p/v8/issues/detail?id=5149 .

Reverts https://codereview.chromium.org/1529303003

R=littledan@chromium.org
R=verwaest@chromium.org

Review-Url: https://codereview.chromium.org/2671553006
Cr-Commit-Position: refs/heads/master@{#44078}