Limit Public-Key-Pins max-age to 60 days

Set the maximum validity period for
a Public-Key-Pins header to 60 days as suggested by
http://tools.ietf.org/html/rfc7469#section-4.1

BUG=523654
TEST=net_unittests

Review URL: https://codereview.chromium.org/1733973004

Cr-Commit-Position: refs/heads/master@{#378231}
3 files changed