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".

[1] https://w3c.github.io/webappsec-csp/#violation-disposition
[2] https://w3c.github.io/webappsec-csp/#policy-disposition

Intent to Ship - https://groups.google.com/a/chromium.org/d/topic/blink-dev/ip4dmVNr15Y/discussion

BUG=646021
R=mkwst@chromium.org

Review-Url: https://codereview.chromium.org/2331213002
Cr-Commit-Position: refs/heads/master@{#425438}