[Linux] Fixed the regression issue that failed to start xwalk from comma...

...nd line.

In the patch 053869eb we set the url_spec into manifest using
keys::kStartURLKey instead of keys::kURLKey in
ApplicationData::Create, and we forget to replace the
Application::URLKey by Application::StartURLKey when launching
application from url in ApplicationSystem::LaunchWithCommandLineParam,
in this case we would fail to lunch xwalk on Linux from command line
with a startup url as its argument.