Revert "Xwalk can not be launched with empty url."

This has caused 6 tests to fail on Linux:

xwalk_browsertest:
        LoadGeolocationPage
        LoadURLAndClose
        TestMultiApp
xwalk_extensions_browsertest:
        CreateNewRuntimeAndNewRP
        OpenLinkInNewRuntimeAndNewRP
        OpenLinkInNewRuntimeAndSameRP

This reverts commit 723ccbedc0e0111e3da212b3b2831f59ac6ad5a9.
Merge pull request #1623 from rakuco/revert-1621

Revert "Xwalk can not be launched with empty url."