Part 5.1:  Is policy list subsumed under subsuming policy?

In this CL we start considering subsumtpion on  MediaListDirective level.
The logic is straightforward: we required exact match for the specified
plugin types.
Two steps:
- Normalize: find effective list of allowed plugin types
- Check for subsumption.

BUG=647588

Review-Url: https://codereview.chromium.org/2541843002
Cr-Commit-Position: refs/heads/master@{#437066}
4 files changed