From the promotion code point of view, a ThinString being forwarded to
an ExternalString is exactly the same as an actual ExternalString being
forwarded to its promoted copy. These changes provide a way to disambiguate
both scenarios since they are different for external memory accounting.

Bug: chromium:867902
Change-Id: I6fd56ee5e0f8900318466108273ab26e936eb439
Reviewed-on: https://chromium-review.googlesource.com/1152975
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54790}