ApplicationFileUtil: Stop leaking |prev_value|.

Stop allocating memory for a base::DictionaryValue() that is immediately
going to be overwritten, as the initial value then leaks.

CID=194508
CID=194777
BUG=XWALK-2928
Merge pull request #2611 from rakuco/CID-194508

ApplicationFileUtil: Stop leaking |prev_value|.