This is needed for both bug 1357461 and bug 1273303, where I plan to add smarter
invalidations than what we have now.
Also, it's cleaner.
Ideally I'll move this onto stylist, though that may require extra work to make
it work fine for Servo, so for now let's just do the obvious thing.
Source-Repo: https://github.com/servo/servo
Source-Revision: 5274991545fedd9b7188c40abffb61b4067daa8a
