[misc-usecase][XWALK-1329] Fix issue for failed to switch to Full Screen mode
The iframe need to have 'allowfullscreen' attribute, otherwise the iframe can
not enter fullscreen mode

BUG=XWALK-1329
Merge pull request #301 from kurli/fullscreen

[misc-usecase][XWALK-1329] Fix issue for failed to switch to Full Screen mode