Add GetURLForBrowserTab method to the external file system backend.

chrome/browser/chromeos/fileapi/file_system_backend_delegate.h

Index: chrome/browser/chromeos/fileapi/file_system_backend_delegate.h
diff --git a/chrome/browser/chromeos/fileapi/file_system_backend_delegate.h b/chrome/browser/chromeos/fileapi/file_system_backend_delegate.h
index b11c86a5ddefea8b979f70f08bffa79c3e15430d..c8042952743a9deeb26c54af2095fa5058342d97 100644
--- a/chrome/browser/chromeos/fileapi/file_system_backend_delegate.h
+++ b/chrome/browser/chromeos/fileapi/file_system_backend_delegate.h
@@ -6,9 +6,12 @@
 #define CHROME_BROWSER_CHROMEOS_FILEAPI_FILE_SYSTEM_BACKEND_DELEGATE_H_
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
 #include "webkit/common/fileapi/file_system_types.h"
 
+class GURL;
+
 namespace base {
 class Time;
 }  // namespace base
@@ -55,6 +58,11 @@ class FileSystemBackendDelegate {
   // stay valid until shutdown.
   virtual storage::WatcherManager* GetWatcherManager(
       const storage::FileSystemURL& url) = 0;
+
+  // Called from FileSystemBackend::GetAlternativeURL.
+  virtual void GetAlternativeURL(
+      const storage::FileSystemURL& url,
+      base::Callback<void(const GURL& url)> callback) = 0;
 };
 
 }  // namespace chromeos