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}