This reverts commit 5f2f418d.

Reason for revert: Speculative revert for LayoutTest timeouts

https://ci.chromium.org/buildbot/client.v8.fyi/V8-Blink%20Linux%2064/24596
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8-Blink%20Linux%2064%20-%20future/4707
https://ci.chromium.org/buildbot/client.v8.fyi/V8-Blink%20Linux%2064%20(dbg)/12467

Original change's description:
> [scanner] Rewrite character streams by separating underlying bytestreams from buffering.
> 
> Additionally now we only scan over flat heap strings.
> 
> Change-Id: Ic449b19aecd7fc3f283a04a3df6a39772d471565
> Reviewed-on: https://chromium-review.googlesource.com/1125854
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54224}

TBR=marja@chromium.org,verwaest@chromium.org

Change-Id: Ica3026f318a85ec6bb24a38a8cd998f12c146d7e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1126819Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54231}