- RegExp.prototype.toString() doesn't have any special handling of
  RegExp instances and simply calls the source and flags getters
- Use the original values of global and sticky, rather than based
  on the current flag getters, as specified in
  https://github.com/tc39/ecma262/pull/494

R=yangguo@chromium.org,adamk
LOG=Y
BUG=v8:4602

Review URL: https://codereview.chromium.org/1846303002

Cr-Commit-Position: refs/heads/master@{#35225}