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Support is compliant with RFC 5280 and supports all the policy
extensions specified therein:

 * Inhibit Any Policy
 * Policy Constraints
 * Policies
 * Policy Mappings

Testing is done solely using the PKITS test suite, which has fairly good
coverage of these extensions:

  4.8 (Certificate Policies)
  4.9 (Require Explicit Policy)
  4.10 (Policy Mappings)
  4.11 (Inhibit Policy Mapping)
  4.12 (Inhibit Any Policy)

BUG=634456,634453,634452

Review-Url: https://codereview.chromium.org/2903283002
Cr-Commit-Position: refs/heads/master@{#476513}