R1:d5befb1a0ea3

Block use of "<base />" in the Content Security Policy

Summary: Ref T4340. We don't use "<base />" so we can safely block it.

Test Plan: Injected "<base />" into a page, saw an error in the console showing that the browser had blocked it.

Maniphest Tasks: T4340

Differential Revision: https://secure.phabricator.com/D19158