Implement GetDisplayed on android M+

chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java

Index: chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java
diff --git a/chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java b/chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java
index bd9b9b7782419a4368897615a657806b6f17dd7d..0d774e2dfc72530e398e2988d62f2196df336e16 100644
--- a/chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java
+++ b/chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/notifications/MockNotificationManagerProxy.java
@@ -5,6 +5,7 @@
 package org.chromium.chrome.test.util.browser.notifications;
 
 import android.app.Notification;
+import android.service.notification.StatusBarNotification;
 
 import org.chromium.chrome.browser.notifications.NotificationManagerProxy;
 
@@ -101,6 +102,11 @@ public class MockNotificationManagerProxy implements NotificationManagerProxy {
         mMutationCount++;
     }
 
+    @Override
+    public StatusBarNotification[] getActiveNotifications() {
+        return null;
+    }
+
     private static String makeKey(int id, @Nullable String tag) {
         String key = Integer.toString(id);
         if (tag != null) key += KEY_SEPARATOR + tag;