Implement GetDisplayed on android M+

chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java

Index: chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java b/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java
index fd9ae67777b09c0f221c31ec28ad941ed8062056..f601d75b5cbf3882f523b101d521e9aca9028532 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationManagerProxy.java
@@ -4,7 +4,10 @@
 
 package org.chromium.chrome.browser.notifications;
 
+import android.annotation.TargetApi;
 import android.app.Notification;
+import android.os.Build;
+import android.service.notification.StatusBarNotification;
 
 /**
  * A proxy for the Android Notification Manager. This allows tests to be written without having to
@@ -18,4 +21,7 @@ public interface NotificationManagerProxy {
     void cancelAll();
     void notify(int id, Notification notification);
     void notify(String tag, int id, Notification notification);
+
+    @TargetApi(Build.VERSION_CODES.M)
+    StatusBarNotification[] getActiveNotifications();
 }