Add `disposition` to  SecurityPolicyViolationEvent

Recently, Content Security Policy specification introduced disposition
of violation [1]. This change extends the SecurityPolicyVioaltionEvent with
`disposition` property, that indicates the disposition of the violated policy.
Property value is a read-only string that corresponds to the policy
disposition [2], and can be either "enforce" or "report".


Intent to Ship -


Cr-Commit-Position: refs/heads/master@{#425438}
45 files changed