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.