[Runtime] Fix launch issue for widget applications that don't have
'content' element in config.xml.

Per Widget spec, the occurrences of 'content' element can be zero or one.
If there is no 'content' in config.xml, we should still be able to launch
widget application by the default 'index.html' path

BUG=XWALK-1109
Merge pull request #1667 from shaochangbin/content-src-issue

[Runtime] Fix launch issue for widget applications that don't have 'content' element in config.xml.