remove duplicate friend declaration

This unbreaks GCC builds after https://codereview.chromium.org/2036403002

BUG=587025
TBR=lfg@chromium.org, dcheng@chromium.org

Review-Url: https://codereview.chromium.org/2121953003
Cr-Commit-Position: refs/heads/master@{#403795}
1 file changed