[webapi] Remove 3 tests for CSS3 Background due to spec update

- box-decoration-break has been removed from new spec
http://www.w3.org/TR/2014/WD-css3-background-20140204/

Impacted tests(approved): new 0, update 0, delete 3
Unit test platform: [Android][IVI]
Unit test result summary: pass 538, fail 0, block 0
[wrt]XWALK-1731 Update test.xml to approve the backend tcs

[Detailed commit message (if have)]
Impacted TCs num(approved): New 0, update 3 , deleted 0
Unit test platform: [tizen]
Test Result: Pass 3, Fail 0
Merge pull request #989 from kangxu/kangxu/webapi/tct-backgrounds-css3-tests/removeCases

[webapi] Remove 3 tests for CSS3 Background due to spec update
Merge pull request #1011 from linshen/shenlin/update-1731-to-approved

[wrt]XWALK-1731 Update test.xml to approve the backend tcs