Add a property to the metric object to help identify bfcache restore instances

[philipwalton]

In order for performance metrics to properly capture real-user experience, they also need to be reported in cases where a browser optimization made the experience really good.

Not reporting in these cases could result in overall regressions in the aggregate numbers as browsers improve. However, good experiences due to browsers optimizations are sometimes not relevant to web developers trying to diagnose specific performance problems.

The ideal solution to both of these concerns is to annotate when these browsers optimizations are happening, so developers can exclude them from reporting in situations where those experiences are not relevant (e.g. when trying to understand slow network response times).

In this library we can add something like a navigationType or visitTypeproperty to the Metric object and set its value to back_forward_cache (name TBD) for bfcache restores or the browser-reported navigationType in all other cases.