This CL lays down the foundation for future work on the subresource filter
component. See the 'intent to implement' posted to security-dev@, under the
title "Extend Safe Browsing protection to embedded sub-resources".

In response to finding that many users proceed through Safe Browsing warning
interstitials that are displayed when the site ahead contains deceptive
embedded content, we would like provide another layer of protection for these
users. After the interstitial has been shown, clicked through and when the
actual content on the page is loaded, this component will be used to filter
network requests for some embedded sub-resources on these sites.

This CL concerns itself merely with adding the skeleton of the architecture,
plus a mock filter to be used in browsertests that disallows all subresource
loads. The component is disabled and has no effect in production code.

BUG=609747
TBR=tsepez@chromium.org

Review-Url: https://codereview.chromium.org/2022783002
Cr-Commit-Position: refs/heads/master@{#398534}