Move WebappRegistry warmup to async task.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
index 2f4d265096609eb46c811a4e1c24b2f8e99fc664..9ee9075312b3efe99ab0b31b6abc1f194e7ce46e 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
@@ -49,7 +49,11 @@ public class WebappRegistry {
     /** Represents a period of 13 weeks in milliseconds */
     static final long WEBAPP_UNOPENED_CLEANUP_DURATION = TimeUnit.DAYS.toMillis(13L * 7L);
 
-    private static volatile WebappRegistry sInstance;
+    /** Initialization-on-demand holder. This exists for thread-safe lazy initialization. */
+    private static class Holder {
+        // Not final for testing.
+        private static WebappRegistry sInstance = new WebappRegistry();
+    }
 
     private HashMap<String, WebappDataStorage> mStorages;
     private SharedPreferences mPreferences;
@@ -62,34 +66,37 @@ public class WebappRegistry {
         void onWebappDataStorageRetrieved(WebappDataStorage storage);
     }
 
+    private WebappRegistry() {
+        mPreferences = openSharedPreferences();
+        mStorages = new HashMap<>();
+    }
+
     /**
-     * Returns the singleton WebappRegistry instance. Creates the instance if necessary.
+     * Returns the singleton WebappRegistry instance. Creates the instance on first call.
      */
     public static WebappRegistry getInstance() {
-        if (sInstance == null) sInstance = new WebappRegistry();
-        return sInstance;
+        return Holder.sInstance;
     }
 
     /**
-     * Warm up the WebappRegistry and a specific WebappDataStorage SharedPreferences. This static
-     * method can be called on any thread, so it must not initialize sInstance.
+     * Warm up the WebappRegistry and a specific WebappDataStorage SharedPreferences.
      * @param id The web app id to warm up in addition to the WebappRegistry.
      */
     public static void warmUpSharedPrefsForId(String id) {
-        sInstance.initStorages(id, false);
+        getInstance().initStorages(id, false);
     }
 
     /**
-     * Warm up the WebappRegistry and all WebappDataStorage SharedPreferences. This static method
-     * can be called on any thread, so it must not initialize sInstance.
+     * Warm up the WebappRegistry and all WebappDataStorage SharedPreferences.
      */
     public static void warmUpSharedPrefs() {
-        sInstance.initStorages(null, false);
+        getInstance().initStorages(null, false);
     }
 
+    @VisibleForTesting
     public static void refreshSharedPrefsForTesting() {
-        sInstance = new WebappRegistry();
-        sInstance.initStorages(null, true);
+        Holder.sInstance = new WebappRegistry();
+        getInstance().initStorages(null, true);
     }
 
     /**
@@ -264,11 +271,6 @@ public class WebappRegistry {
                 REGISTRY_FILE_NAME, Context.MODE_PRIVATE);
     }
 
-    private WebappRegistry() {
-        mPreferences = openSharedPreferences();
-        mStorages = new HashMap<String, WebappDataStorage>();
-    }
-
     private void initStorages(String idToInitialize, boolean replaceExisting) {
         Set<String> webapps =
                 mPreferences.getStringSet(KEY_WEBAPP_SET, Collections.<String>emptySet());