[Runtime] Fix WGT config.xml parsing error if one type of tag has more than three
instances.

When parsing config.xml, if one type of element has more than three
instances, the ListValue will be destroyed by excess invoking Value::Set
method, this will cause the crash when visiting the list.

BUG=XWALK-1142
Merge pull request #1703 from takethathe/fix-XWALK-1142

[Runtime] Fix WGT config.xml parsing error if one type of tag has more than three instances.