[Android Refactor] Merge WebappDataStorage#LAST_USED_UNSET and LAST_USED_INVALID

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.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.SharedPreferences;
 import android.content.pm.PackageManager;
 import android.os.AsyncTask;
@@ skipping 99 matching lines @@
         new AsyncTask<Void, Void, WebappDataStorage>() {
             @Override
             protected final WebappDataStorage doInBackground(Void... nothing) {
                 // Create the WebappDataStorage on the background thread, as this must create and
                 // open a new SharedPreferences.
                 return WebappDataStorage.open(webappId);
             }
 
             @Override
             protected final void onPostExecute(WebappDataStorage storage) {
-                // Guarantee that last used time != WebappDataStorage.LAST_USED_INVALID. Must be
-                // run on the main thread as SharedPreferences.Editor.apply() is called.
+                // Update the last used time in order to prevent
+                // {@link WebappRegistry@unregisterOldWebapps()} from deleting the
+                // WebappDataStorage. Must be run on the main thread as
+                // SharedPreferences.Editor.apply() is called.
                 mStorages.put(webappId, storage);
                 mPreferences.edit().putStringSet(KEY_WEBAPP_SET, mStorages.keySet()).apply();
                 storage.updateLastUsedTime();
                 if (callback != null) callback.onWebappDataStorageRetrieved(storage);
             }
         }.execute();
     }
 
     /**
      * Returns the WebappDataStorage object for webappId, or null if one cannot be found.
@@ skipping 147 matching lines @@
                 }
             }
         } else {
             if (webapps.contains(idToInitialize)
                     && (replaceExisting || !mStorages.containsKey(idToInitialize))) {
                 mStorages.put(idToInitialize, WebappDataStorage.open(idToInitialize));
             }
         }
     }
 }