src/js/regexp.js was one of the few files that was left in sloppy
mode. The ES2017 draft specification requires that writes to
lastIndex throw when the property is non-writable, and test262
tests enforce this behavior. This patch puts that file in strict
mode.

BUG=v8:4504
R=yangguo@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/1776883005

Cr-Commit-Position: refs/heads/master@{#34801}