Mark kKnownRootCertSHA256Hashes as const

Marking kKnownRootCertSHA256Hashes as const puts it into the read-only
data segment which has various minor advantages such as making data
corruption impossible and guaranteeing that the data will always be
in shareable pages.

R=rsleevi@chromium.org
BUG=677351

Review-Url: https://codereview.chromium.org/2608643002
Cr-Commit-Position: refs/heads/master@{#440914}
1 file changed