added chromium support for devTools on linux

Summary:
<details>
  Thanks for submitting a PR! Please read these instructions carefully:

  - [ ] 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
![screenshot](https://user-images.githubusercontent.com/13065528/27481217-ceaf5e58-581b-11e7-976f-75c107596ad3.png)
Closes https://github.com/facebook/react-native/pull/14696

Differential Revision: D5398564

Pulled By: hramos

fbshipit-source-id: 151f83b549492c8716a248eb16f7e24c5658b32e

local-cli/server/util/launchChrome.js

@@ -11,6 +11,18 @@
 'use strict';
 
 const opn = require('opn');
+const execSync = require('child_process').execSync;
+
+function commandExistsUnixSync (commandName, callback) {
+  try {
+    var stdout = execSync('command -v ' + commandName +
+          ' 2>/dev/null' +
+          ' && { echo >&1 \'' + commandName + ' found\'; exit 0; }');
+    return !!stdout;
+  } catch (error) {
+    return false;
+  }
+}
 
 function getChromeAppName(): string {
   switch (process.platform) {
@@ -18,6 +30,12 @@ function getChromeAppName(): string {
     return 'google chrome';
   case 'win32':
     return 'chrome';
+  case 'linux':
+    if (commandExistsUnixSync('google-chrome')) {
+      return 'google-chrome';
+    } else {
+      return 'chromium';
+    }
   default:
     return 'google-chrome';
   }