This reverts commit 2df5e7a7.

Reason for revert: Mystery crashes https://bugs.chromium.org/p/chromium/issues/detail?id=838805

Original change's description:
> [parser] Slice the source string where possible
> 
> When internalizing string literals (for quoted strings or property names),
> try to create a sliced string of the source string rather than allocating
> a copy of the bytes.
> 
> This will not work for string literals that contain escapes (e.g. unicode
> escapes), and currently does not support two-byte strings.
> 
> Bug: chromium:818642
> Change-Id: I686e5ad36baecd1a84ce5e124118431249b6c980
> Reviewed-on: https://chromium-review.googlesource.com/1010282
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52898}

TBR=marja@chromium.org,yangguo@chromium.org,jarin@chromium.org,mlippautz@chromium.org,leszeks@chromium.org,verwaest@chromium.org

Change-Id: I598b6668c43a3e843e2dd8e60852b2b2f3461954
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:818642
Reviewed-on: https://chromium-review.googlesource.com/1039885
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52919}