This patch implements one aspect of ES2015 RegExp subclassing:
String.prototype.replace is separated into two parts, a method on
RegExp.prototype in case the first argument is a RegExp, and the
String.prototype.replace method, which handles the string pattern
case. This separation is described in the ES2015 specification.
Most of the patch is simply moving code from string.js to regexp.js.

R=yangguo
LOG=Y
BUG=v8:4343

Review URL: https://codereview.chromium.org/1590673002

Cr-Commit-Position: refs/heads/master@{#33393}