Emphasize this code handles ephemerons (key-value pairs)
and not ephemeral objects (short-lived objects).

Bug: chromium:844008
Change-Id: I1dbf97f69ea5d17b1eb446d92dffef0cc3d5d08f
Reviewed-on: https://chromium-review.googlesource.com/1095101Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53628}