DevTools: React Native: Support reload-and-profile

[bvaughn]

React DevTools v4 adds a new reload and profile feature to measure perf for application "mount" (although it required a bit of hacking). I don't feel knowledgeable enough about React Native to tackle it, so my current plan is to just not support this feature for RN.

If we did decide to support it,I think we would need to solve the following:

1. A reload hook on the backend that worked for all bundle types (not just DEV).
2. Some assurance that the backend will be injected/initialized before the first mount/commit (or a mechanism to delay the first commit, like we do in the browser).
3. Some way for third party code to request a production+profiling build (similar to how DOM does it).
4. A sync storage mechanism (or some other way for DevTools could leave a flag for itself so it knows to begin profiling immediately after reload+connection).

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Originally reported via bvaughn/react-devtools-experimental#336