[webapi][XWALK-2334] Remove a pop-up during executing "CSS3BG_background-attach$

Impacted tests(approved): New 0, Update 1, Delete 0
Unit test platform: [Android]
Unit test result summary: Pass 1, Fail 0, Block 0
Merge pull request #723 from minxhuang/hm/webapi/xwalk-2334-hotfix

[webapi][XWALK-2334] Remove pop-up for backgrounds