Even though most compilers accept the method to be private, gcc 4.8
complains about this (which seems to be correct). Thus we make this
method protected.

R=franzih@chromium.org

Change-Id: Ia49b2ddebe1ced7529d4943107a76a909c355b73
Reviewed-on: https://chromium-review.googlesource.com/890449Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50931}