As with other code size stats, this doesn't distinguish between live and
dead objects, and doesn't scan the young generation.

Also make ExternalString::is_short() const.

Bug: chromium:837659
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I72815edb719ba61d9727e226ff1da0fc4af22a24
Reviewed-on: https://chromium-review.googlesource.com/1032994
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52955}