[Runtime] Add user href attribute check in widget element handler.

When parsing a widget config.xml file, runtime should make sure the author href
attribute is a valid URI, as it's required in W3C Widget spec. This patch will
fix this.

BUG=XWALK-2995
Merge pull request #2639 from takethathe/XWALK-2995

[Runtime] Add author href attribute check in widget element handler.