Feature request: Show CPU frequency / on-off state

CPU governors make it really hard to see problems with performance on modern mobile devices — the data lies to you ([article](https://medium.com/flutter/performance-testing-of-flutter-apps-df7669bb7df7)). One solution is to scale lock the device, but many people won't have the patience or resources to do it.

The next best thing is to _see_ when CPU shifts into higher gear (or when a previously-disabled CPU is switched on). It would be awesome to show this in the timeline.

// cc @liyuqian