Show graph of % CPU usage in Performance page #760

In the performance page, it would be nice if we could have a top chart (similar to timeline and memory) where we show an overview of the recorded activity. The overview for the performance page would be a graph of % CPU usage.

@bkonyi is this data available from the VM? Perhaps we could query percent CPU usage every ~100ms or so and graph it that way, or - even better - maybe we could hook up a stream and listen for percent usage events from the VM.