[ios] Updated -[CRWCertVerificationController shutDown] comments.
[ios] Updated -[CRWCertVerificationController shutDown] comments.

|shutDown| does not cancel pending verification requests since:
https://codereview.chromium.org/1392143003/
so removed comment which said that pending verification requests are
cancelled.

BUG=None

Review URL: https://codereview.chromium.org/1412343006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357878}

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