[SystemInfo API] Implement systemInfo.storage.onAvailableCapacityChanged event

chrome/browser/extensions/api/system_info_storage/storage_info_observer.h

Index: chrome/browser/extensions/api/system_info_storage/storage_info_observer.h
diff --git a/chrome/browser/extensions/api/system_info_storage/storage_info_observer.h b/chrome/browser/extensions/api/system_info_storage/storage_info_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..5dd9cc9b6240a9bd1ce88f0f735a009ed51437a4
--- /dev/null
+++ b/chrome/browser/extensions/api/system_info_storage/storage_info_observer.h
@@ -0,0 +1,22 @@
+// Copyright 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_EXTENSIONS_API_SYSTEM_INFO_STORAGE_STORAGE_INFO_OBSERVER_H_
+#define CHROME_BROWSER_EXTENSIONS_API_SYSTEM_INFO_STORAGE_STORAGE_INFO_OBSERVER_H_
+
+namespace extensions {
+
+// Observes the free space changes happening on the storage.
+class StorageInfoObserver {
+ public:
+  virtual ~StorageInfoObserver() {}
+
+  // Called when the storage free space changes.
+  virtual void OnStorageFreeSpaceChanged(const std::string& id,
+                                         double old_value,
+                                         double new_value) = 0;
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_API_SYSTEM_INFO_STORAGE_STORAGE_INFO_OBSERVER_H_