Download notification: Fix a crash that startForeground is not called.

If stopForeground is called between startForegroundService and
startForeground. startForeground will not be called since we clear the
cached service object. This will lead to a crash on O_MR1+.

Android can reuse the service binder object when this happens.

Bug: 1121096
Change-Id: Ic4088af174ca9561714442b29c2e6162995ddc51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429635Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810857}

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

@@ -276,7 +276,6 @@ public class DownloadForegroundServiceManager {
         stopAndUnbindServiceInternal(
                 stopForegroundNotification, mPinnedNotificationId, oldNotification);
 
-        mBoundService = null;
 
         mPinnedNotificationId = INVALID_NOTIFICATION_ID;
     }