[xwalkdriver] Fix get_cookie() always get None

  Fulfill that xwalkdriver can aqcuire specific cookie content by
name of cookie through driver.get_cookie('name') on selenium side.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2437
Merge pull request #11 from wuyuehang/XWAL-2437

[xwalkdriver] Fix get_cookie() always get None