[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
s/allowfullscreen/allowFullScreen/

See the iframe usage in tct-fullscreen-nonw3c-tests/webrunner/index.html