Forward the concrete class used in CRTP to avoid having virtual class to
the outmost declaration.

Bug: chromium:736641
Change-Id: I8ab693b8f75507318a892c1994d9c899ba5a41d1
Reviewed-on: https://chromium-review.googlesource.com/549303
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46253}