Xwalk can not be launched with empty url.

The constructor "GURL()" will create an invalid Url rather than "about:blank".
That causes xwalk failed to be launched with empty url, for example:
command "./xwalk" will be failed.

BUG=https://crosswalk-project.org/jira/browse/XWALK-958
Merge pull request #1621 from iKevinHan/emptyurl

Xwalk can not be launched with empty url.