[Runtime] runtime_download_manager_delegate: Add the check for CreateDirectory().
[Runtime] runtime_download_manager_delegate: Add the check for
CreateDirectory().

The return value of CreateDirectory() was not being checked in
GenerateFilename().

CID=194047
BUG=XWALK-2928

(cherry picked from commit ddb0e68cb304d2ab0696758bfeb8b2cb4d263e5c)

TheDirtyCalvinist/spacewalk,TheDirtyCalvinist/spacewalk,TheDirtyCalvinist/spacewalk,TheDirtyCalvinist/spacewalk,TheDirtyCalvinist/spacewalk,TheDirtyCalvinist/spacewalk