fix(Launcher): Terminate chrome gracefully on Windows (#876)

This patch starts using taskkill program on windows to gracefully
terminate chrome.

Note: this slows down chrome shutdown on Windows in case of using
custom userDataDir. This is because chrome takes some time to shutdown
its operations and leave profile directory in a consistent state.

Fixes #839.

lib/Launcher.js

@@ -129,7 +129,7 @@ class Launcher {
       } else {
         // Terminate chrome gracefully.
         if (process.platform === 'win32')
-          chromeProcess.kill();
+          childProcess.execSync(`taskkill /pid ${chromeProcess.pid}`);
         else
           process.kill(-chromeProcess.pid, 'SIGTERM');
       }