This implements a simple load-tracking system and tracks stylesheet loads as an example of how it fits together. This is a simplified and rebased version of #3714; I do not believe that the main thrust of hsivonen's comments (related to tracking navigation in browsing contexts) affect this part of the work.
r? @Ms2ger
Source-Repo: https://github.com/servo/servo
Source-Revision: 2baa69595e2d57213c34b7e168b60885948fa85b
