Support "display": "fullscreen" for sites added to the home screen.

chrome/android/java/src/org/chromium/chrome/browser/webapps/FullScreenActivity.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.Intent;
 import android.util.Pair;
 import android.view.View;
 import android.view.ViewGroup;
@@ skipping 178 matching lines @@
     /**
      * @return {@link File} pointing at a directory specific for this class.
      */
     protected File getActivityDirectory() {
         return null;
     }
 
     @Override
     protected boolean handleBackPressed() {
         if (mTab == null) return false;
+
+        if (exitFullscreenIfShowing()) return true;
+
         if (mTab.canGoBack()) {
             mTab.goBack();
             return true;
         }
         return false;
     }
 
     @Override
     public void onCheckForUpdate(boolean updateAvailable) {
     }
 }