deps: backport 200315c from V8 upstream

deps: backport 200315c from V8 upstream

Original commit message:

  Make AstRawString deduplication encoding-agnostic.

  R=jkummerow@chromium.org
  BUG=v8:4450
  LOG=N

  Review URL: https://codereview.chromium.org/1411103006

  Cr-Commit-Position: refs/heads/master@{#31624}

Ref: https://github.com/nodejs/node/pull/4160
PR-URL: https://github.com/nodejs/node/pull/4128
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>