gdata: Fix a bug that caused a crash when downloading files

chrome/browser/chromeos/gdata/gdata_file_system.cc

Index: chrome/browser/chromeos/gdata/gdata_file_system.cc
diff --git a/chrome/browser/chromeos/gdata/gdata_file_system.cc b/chrome/browser/chromeos/gdata/gdata_file_system.cc
index ee92b9d2914ba5403ecbf5ec7518471911c88434..41159f7eb884d653f279f397cc1d310cc1cf374d 100644
--- a/chrome/browser/chromeos/gdata/gdata_file_system.cc
+++ b/chrome/browser/chromeos/gdata/gdata_file_system.cc
@@ -773,6 +773,10 @@ void RelayGetDownloadDataCallback(
     const GetDownloadDataCallback& callback,
     GDataErrorCode error,
     scoped_ptr<std::string> download_data) {
+  // Unlike other callbacks, GetDownloadDataCallback is optional, hence it
+  // can be null here.
+  if (callback.is_null())
+    return;
   relay_proxy->PostTask(
       FROM_HERE,
       base::Bind(callback, error, base::Passed(&download_data)));