Summary:
<details>
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- [ ] Explain the **motivation** for making this change.
- [ ] Provide a **test plan** demonstrating that the code is solid.
- [ ] Match the **code formatting** of the rest of the codebase.
- [ ] Target the `master` branch, NOT a "stable" branch.
Please read the [Contribution Guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md) to learn more about contributing to React Native.
</details>
_What existing problem does the pull request solve?_
On Linux, the packager caught an "Google Chrome exited with error: { Error: spawn google-chrome ENOENT}" when trying to launch the devTools because google-chrome is not installed but chromium is.
Thus, this pull request maps the platform Linux with chromium for launching the debugger automatically in the packager
_A good test plan has the exact commands you ran and their output, provides screenshots or videos if the pull request changes UI or updates the website._
- enter on terminal
> react-native start
- launch the app in dev mode with "Debug JS remotely" enabled
- the packager prints "Launching Dev Tools..." and launch chromium with the debugger
Closes https://github.com/facebook/react-native/pull/14696
Differential Revision: D5398564
Pulled By: hramos
fbshipit-source-id: 151f83b549492c8716a248eb16f7e24c5658b32e
3f16aa5559
