[DevTools] add perf regression test page in shell

[mondaychen]

Summary

This PR adds a "perf regression tests" page to react-devtools-shell. This page is meant to be used as a performance sanity check we will run whenever we release a new version or finish a major refactor.
Similar to other pages in the shell, this page can load the inline version of devtools and a test react app on the same page. But this page does not load devtools automatically like other pages. Instead, it provides a button that allows us to load devtools on-demand, so that we can easily compare perf numbers without devtools against the numbers with devtools.

As a first step, this page currently only contain one test: mount/unmount a large subtree. This is to catch perf issues that devtools can cause on the react applications it's running on, which was once a bug fixed in #24863.
In the future, we plan to add:

• more test apps covering different scenarios
• perf numbers within devtools (e.g. initial load)

How did you test this change?

In order to show this test app can actually catch the perf regression it's aiming at, I reverted #24863 locally. Here is the result:

devtools-regression-with-revert.mov

As shown in the video, the time it takes to unmount the large subtree significantly increased after DevTools is loaded.

For comparison, here is how it looks like before the fix was reverted:

about the requestAnimationFrame method

For this test, I used requestAnimationFrame to catch the time when render and commit are done. It aligns very well with the numbers reported by Chrome DevTools performance profiling. For example, in one run, the numbers reported by my method are

They are very close to the numbers reported by Chrome profiling:

<Profiler> is not able to catch this issue here.

If you are aware of a better way to do this, please kindly share with me.

[lunaruan]

Stamping to unblock but If we want to do it like this we probably should write down a test plan somewhere of how to do this