[Extensions] Disable the Extension Framework entirely for --install and --uninstall

As mentioned in https://github.com/crosswalk-project/crosswalk/issues/839,
we shouldn't be bootstrapping the entire BrowserProcess workflow when using
xwalk --install or xwalk --uninstall.

This patch disables the ExtensionService creation and the Extensions
loading and registering as a workaround.

(In fact, XWalkBrowserMainParts::PreMainMessageLoopRun() is begging for a refactor already.)