Introduce a typedef to avoid repeating the function definition multiple
times.

R=ahaas@chromium.org

Change-Id: I9d8a2a9b663f86ce0f6e21edf6d4a6d5ae450efc
Reviewed-on: https://chromium-review.googlesource.com/c/1325963Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57352}