Disable multiple windows to align navigation behavior with upstream.

As Crosswalk enable multiple windows to implement opening url with system
default browser, the navigation by clicking <a target="_blank"> in Cordova
was blocked, so disable multiple windows with XWalkPreferences.

To open urls in system default browser, the InAppBrowser plugin should be
used.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2157
Merge pull request #124 from gaochun/fix

Disable multiple windows to align navigation behavior with upstream.