[api] Launcher: Close gracefully when a userDataDir is specified (#700)

This patch:
- makes `browser.close()` return a promise that resolves when browser gets closed
- starts closing chrome gracefully if a custom `userDataDir` is supplied

Fixes #527

lib/Launcher.js

@@ -84,9 +84,14 @@ class Launcher {
       chromeProcess.stderr.pipe(process.stderr);
     }
 
-    // Cleanup as processes exit.
-    if (temporaryUserDataDir)
-      chromeProcess.once('close', () => removeSync(temporaryUserDataDir));
+    const waitForChromeToClose = new Promise(fulfill => {
+      chromeProcess.once('close', () => {
+        // Cleanup as processes exit.
+        if (temporaryUserDataDir)
+          removeSync(temporaryUserDataDir);
+        fulfill();
+      });
+    });
 
     const listeners = [ helper.addEventListener(process, 'exit', killChrome) ];
     if (options.handleSIGINT !== false)
@@ -102,12 +107,29 @@ class Launcher {
       throw e;
     }
 
+    /**
+     * @return {Promise}
+     */
     function killChrome() {
       helper.removeEventListeners(listeners);
-      if (process.platform === 'win32')
-        childProcess.execSync(`taskkill /pid ${chromeProcess.pid} /T /F`);
-      else
-        process.kill(-chromeProcess.pid);
+      if (temporaryUserDataDir) {
+        // Force kill chrome.
+        if (process.platform === 'win32')
+          childProcess.execSync(`taskkill /pid ${chromeProcess.pid} /T /F`);
+        else
+          process.kill(-chromeProcess.pid, 'SIGKILL');
+        // Attempt to remove temporary profile directory to avoid littering.
+        try {
+          removeSync(temporaryUserDataDir);
+        } catch (e) { }
+      } else {
+        // Terminate chrome gracefully.
+        if (process.platform === 'win32')
+          chromeProcess.kill();
+        else
+          process.kill(-chromeProcess.pid, 'SIGTERM');
+      }
+      return waitForChromeToClose;
     }
   }