Fix for tct-extra-html5-tests/base64.html

[Problem] The following test cases from tct-extra-html5-tests/base64 fail
    - base64_atob_ab_cd
    - base64_atob_abcd_space
    - base64_atob_space_abcd
[Cause] Those test cases fails since  Chromium implements the newest
        standard of base64 feature but the test has not been updated yet.

[Solution] Apply latest changes of base64.html from
           http://w3c-test.org/html/webappapis/atob/base64.html

BUG=XWALK-2227
Merge pull request #616 from jizydorczyk/XWALK-2227

Fix for tct-extra-html5-tests/base64.html