Observer callbacks should be deferred until after activation

[philipwalton]

For prerendered pages, in Chrome's current implementation it will always defer running any Performance Observer callbacks until after the page has been activated.

However, I'm not sure if we can guarantee that will always be the case (spec changes, browser bugs, etc.), so I think it makes sense to defer handling any Performance Observer callbacks until after activation.

If callbacks are handled prior to activation then activationStart will always be 0, and thus any calculations made using it will be incorrect.