This reverts commit 7bff339e.

Reason for revert: Breaks autoroll, see bug.

Bug: v8:7944

Original change's description:
> Reland "[heap] Added External Strings to external memory accounting."
> 
> This is a reland of 5863c0b6
> 
> Original change's description:
> > [heap] Added External Strings to external memory accounting.
> > 
> > Bug: chromium:845409
> > Change-Id: I3fe2b294f6e038d77787cf0870d244ba7cc20550
> > Reviewed-on: https://chromium-review.googlesource.com/1118164
> > Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
> > Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#54110}
> 
> Bug: chromium:845409
> Change-Id: Ied341ec6268000343d2a577b22f2a483460b01f5
> Reviewed-on: https://chromium-review.googlesource.com/1121736
> Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
> Reviewed-by: Peter Marshall <petermarshall@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54410}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org,petermarshall@chromium.org,rfbpb@google.com

Change-Id: Ie55586e84f44a2d83c7f97110d60abb86f0730c5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:845409
Reviewed-on: https://chromium-review.googlesource.com/1136312Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54428}