QUIC - minor cleanup of duplicated code in ProofVerifierChromiumTest.
QUIC - minor cleanup of duplicated code in ProofVerifierChromiumTest.

R=rch@chromium.org

Review URL: https://codereview.chromium.org/1458883007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360655}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk