Implement SignedCertificateTimestamp::isolatedCopy

As ResourceResponse::SignedCertificateTimestamp contains String members, it
should have isolatedCopy function and that function should be called in
ResourceResponse::copyData.

BUG=626809

Review-Url: https://codereview.chromium.org/2135143002
Cr-Commit-Position: refs/heads/master@{#404962}
4 files changed