Fix download notification buttons not working in WebLayer.

The BroadcastReceiver needs to be public, which regressed in r757061.

Bug: 1025603
Change-Id: I683936cd4f11af80188d7e7109c55a68af06d800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141222
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757441}

weblayer/public/java/org/chromium/weblayer/BroadcastReceiver.java

@@ -13,7 +13,7 @@ import org.chromium.weblayer_private.interfaces.ObjectWrapper;
 /**
  * Listens to events from WebLayer-spawned notifications.
  */
-class BroadcastReceiver extends android.content.BroadcastReceiver {
+public class BroadcastReceiver extends android.content.BroadcastReceiver {
     @Override
     public void onReceive(Context context, Intent intent) {
         try {