Check null intent when launching DownloadNotificationService

When onStartCommand() is called, it is possible that the intent is null.
According to the SDK page, service must take care of the null check.

BUG=587399

Review URL: https://codereview.chromium.org/1708913002

Cr-Commit-Position: refs/heads/master@{#376048}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadNotificationService.java

@@ -387,6 +387,7 @@ public class DownloadNotificationService extends Service {
      * @return true if the intent requires actions, or false otherwise.
      */
     static boolean isDownloadOperationIntent(Intent intent) {
+        if (intent == null) return false;
         if (!ACTION_DOWNLOAD_CANCEL.equals(intent.getAction())
                 && !ACTION_DOWNLOAD_RESUME.equals(intent.getAction())
                 && !ACTION_DOWNLOAD_PAUSE.equals(intent.getAction())) {