Remove mActivity from Tab and look at windowAndroid for tab

chrome/android/java/src/org/chromium/chrome/browser/webapps/FullScreenActivityTab.java

 // Copyright 2015 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.webapps;
 
 import android.content.Context;
 import android.content.Intent;
 import android.net.Uri;
 import android.os.Bundle;
@@ skipping 297 matching lines @@
 
     @Override
     protected FullScreenTabWebContentsDelegateAndroid createWebContentsDelegate() {
         return new FullScreenTabWebContentsDelegateAndroid();
     }
 
     private class FullScreenTabWebContentsDelegateAndroid
             extends TabChromeWebContentsDelegateAndroid {
         @Override
         public void activateContents() {
-            if (!(mActivity instanceof WebappActivity)) return;
+            if (!(getActivity() instanceof WebappActivity)) return;
 
-            WebappInfo webappInfo = ((WebappActivity) mActivity).getWebappInfo();
+            WebappInfo webappInfo = ((WebappActivity) getActivity()).getWebappInfo();
             String url = webappInfo.uri().toString();
 
             // Create an Intent that will be fired toward the WebappLauncherActivity, which in turn
             // will fire an Intent to launch the correct WebappActivity.  On L+ this could probably
             // be changed to call AppTask.moveToFront(), but for backwards compatibility we relaunch
             // it the hard way.
             Intent intent = new Intent();
             intent.setAction(WebappLauncherActivity.ACTION_START_WEBAPP);
-            intent.setPackage(mActivity.getPackageName());
+            intent.setPackage(getApplicationContext().getPackageName());
             webappInfo.setWebappIntentExtras(intent);
 
-            intent.putExtra(ShortcutHelper.EXTRA_MAC, ShortcutHelper.getEncodedMac(mActivity, url));
+            intent.putExtra(ShortcutHelper.EXTRA_MAC,
+                    ShortcutHelper.getEncodedMac(getActivity(), url));
             intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
             getApplicationContext().startActivity(intent);
         }
     }
 }