Fix window.performance.now()

[Manishearth]

No description provided.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3506

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[jdm]

Looks good!

[mbrubeck]

dom/performance.rs:53:9: 53:78 error: mismatched types: expected `f64`, found `u64` (expected f64, found u64)
dom/performance.rs:53         (time::precise_time_ns() - navStart) * 1000000 as DOMHighResTimeStamp
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
dom/window.rs:385:39: 385:62 error: mismatched types: expected `f64`, found `u64` (expected f64, found u64)
dom/window.rs:385             navigation_start_precise: time::precise_time_ns(),