Report metrics from windows.performance

[jessewgibbs]

Originally suggested in https://app.intercom.io/a/apps/rlisg82q/inbox/inbox/487663/conversations/10894526158.

It would be great to get a rollbar automatically if [window.performance.memory] goes over a certain amount.

[waltjones]

window.performance.memory is Chrome only, and the standardized window.performance data doesn't include a memory measurement. (Chrome could reasonably remove this feature at any time.) Do we want to proceed anyway?

[brianr]

@waltjones I'd say yes. In general, I think we can have features that work in only the environments where the data is available - applying that here, this kind of message would of course only be able to fire in browsers that provide the data.