This change fixes two classes of problems with the js-tests:

The way we were testing the router on the diff viewer didn't work

   with the latest Backbone router/history code. In particular, it's no

   longer possible to use the router without having first called

Backbone.history.start. Fixing this without causing all sorts of

   browser location mischief involved changing the spy to a much lower

   level, at the window.history level, and then bypassing a check that

   ensured that the target navigation URL matched the history root.
On Chrome, some calls to $.css('display') are now returning an

   empty string. This breaks some tests that expected it to return

   "block" (which is the case on Firefox). I've generalized the tests to

   check both.

Ran js-tests on Chrome and Firefox.