[SystemInfo API] Implement storage.getAvailableCapacity API

chrome/browser/extensions/api/system_storage/storage_info_provider.h

Index: chrome/browser/extensions/api/system_storage/storage_info_provider.h
diff --git a/chrome/browser/extensions/api/system_storage/storage_info_provider.h b/chrome/browser/extensions/api/system_storage/storage_info_provider.h
index 859ea87ce182cf2c92db8d385b7991f0c7d2b2ec..6883deb47561ae4914c4c1089d2271d3aded85f1 100644
--- a/chrome/browser/extensions/api/system_storage/storage_info_provider.h
+++ b/chrome/browser/extensions/api/system_storage/storage_info_provider.h
@@ -35,6 +35,9 @@ typedef std::vector<linked_ptr<
 
 class StorageInfoProvider : public SystemInfoProvider {
  public:
+  typedef base::Callback<void(const std::string&, double)>
+      GetStorageFreeSpaceCallback;
+
   // Get the single shared instance of StorageInfoProvider.
   static StorageInfoProvider* Get();
 
@@ -43,6 +46,9 @@ class StorageInfoProvider : public SystemInfoProvider {
   virtual void InitializeProvider(const base::Closure& do_query_info_callback)
       OVERRIDE;
 
+  virtual double GetStorageFreeSpaceFromTransientIdOnFileThread(
+      const std::string& transient_id);
+
   const StorageUnitInfoList& storage_unit_info_list() const;
 
   static void InitializeForTesting(scoped_refptr<StorageInfoProvider> provider);