DevTools: React Native: Support reload-and-profile

React DevTools v4 adds a new reload and profile feature to measure perf for application "mount" ([although it required a bit of hacking](https://github.com/bvaughn/react-devtools-experimental/pull/35)). 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](https://fb.me/react-profiling)). 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). --- Originally reported via https://github.com/bvaughn/react-devtools-experimental/issues/336