[NTP Snippets] Schedule periodic fetching

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetsController.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.ntp.snippets;
 
 import android.content.Context;
 
 import org.chromium.base.ThreadUtils;
+import org.chromium.base.VisibleForTesting;
 import org.chromium.chrome.browser.profiles.Profile;
 import org.chromium.chrome.browser.signin.SigninManager;
 import org.chromium.chrome.browser.signin.SigninManager.SignInStateObserver;
 
 /**
  * The main controller for calling into the native snippets component to fetch snippets
  */
 public class SnippetsController implements SignInStateObserver {
     private static SnippetsController sInstance;
 
     private long mNativeSnippetsController;
 
     public SnippetsController(Context applicationContext) {
-        SigninManager.get(applicationContext).addSignInStateObserver(this);
+        // |applicationContext| can be null in tests.
+        if (applicationContext != null) {
+            SigninManager.get(applicationContext).addSignInStateObserver(this);
+        }
     }
 
     /**
      * Fetches new snippets
      *
      * @param overwrite true if an update to the current snippets should be forced, and false if
      * snippets should be downloaded only if there are no existing ones.
      */
     public void fetchSnippets(boolean overwrite) {
         nativeFetchSnippets(Profile.getLastUsedProfile(), overwrite);
@@ skipping 14 matching lines @@
     }
 
     @Override
     public void onSignedIn() {
         fetchSnippets(true);
     }
 
     @Override
     public void onSignedOut() {}
 
+    @VisibleForTesting
+    public static void setInstanceForTesting(SnippetsController instance) {
+        sInstance = instance;
+    }
+
     private native void nativeFetchSnippets(Profile profile, boolean overwrite);
 }