[behavior][xwalk-1945] Fix failed to switch full screen

- The iframe need to have 'allowfullscreen' attribute, otherwise the iframe can
not enter fullscreen mode

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Android][Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #387 from haoxli/haoxli/behavior/xwalk-1945

[behavior][xwalk-1945] Fix failed to switch full screen