Prepare ExternalPolicyDataUpdater and ResourceCache for blocking pool

chrome/browser/policy/cloud/resource_cache.h

 // Copyright (c) 2013 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.
 
 #ifndef CHROME_BROWSER_POLICY_CLOUD_RESOURCE_CACHE_H_
 #define CHROME_BROWSER_POLICY_CLOUD_RESOURCE_CACHE_H_
 
 #include <map>
 #include <set>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/memory/ref_counted.h"
+
+namespace base {
+class SequencedTaskRunner;
+}
 
 namespace policy {
 
 // Manages storage of data at a given path. The data is keyed by a key and
 // a subkey, and can be queried by (key, subkey) or (key) lookups.
 // The contents of the cache have to be manually cleared using Delete() or
 // PurgeOtherSubkeys().
-// Instances of this class can be created on any thread, but from then on must
-// be always used from the same thread, and it must support file I/O.
-class ResourceCache : public base::NonThreadSafe {
+// The class can be instantiated on any thread but from then on, it must be
+// accessed via the |task_runner| only. The |task_runner| must support file I/O.
+class ResourceCache {
  public:
-  explicit ResourceCache(const base::FilePath& cache_path);
+  explicit ResourceCache(const base::FilePath& cache_path,
+                         scoped_refptr<base::SequencedTaskRunner> task_runner);
   virtual ~ResourceCache();
 
   // Stores |data| under (key, subkey). Returns true if the store suceeded, and
   // false otherwise.
   bool Store(const std::string& key,
              const std::string& subkey,
              const std::string& data);
 
   // Loads the contents of (key, subkey) into |data| and returns true. Returns
   // false if (key, subkey) isn't found or if there is a problem reading the
@@ skipping 28 matching lines @@
   // and returns whether the parent directory of this file exists. If
   // |allow_create_key| is |true|, the directory is created if it did not exist
   // yet. This method does not check whether the file at |path| exists or not.
   bool VerifyKeyPathAndGetSubkeyPath(const std::string& key,
                                      bool allow_create_key,
                                      const std::string& subkey,
                                      base::FilePath* subkey_path);
 
   base::FilePath cache_dir_;
 
+  // Task runner that |this| runs on.
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(ResourceCache);
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_POLICY_CLOUD_RESOURCE_CACHE_H_