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This is a pure interface so it can inline the dtor and doesn't need to
hide the ctor.

Bug: none
Test: compile
Change-Id: Idbf3cd3ca8397b3c0883f5e7759151ff9d29ab55
Reviewed-on: https://chromium-review.googlesource.com/c/1283476
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600970}