[wrt][xwalk-2715] Update 12 TCs to packertoolmanu

Impacted TCs num(approved): New 0, Update 12, Delete 0
Unit test Platform: Android
Android test result summary: Pass 12, Fail 0, Block 0
Merge pull request #1103 from wanghongjuan/wanghongjuan/wrt/xwalk-2715

[wrt][xwalk-2715] Update 12 TCs to packertoolmanu
[webapi] Improve tests purpose description for File Writer API
Merge pull request #1105 from kangxu/kangxu/webapi/tct-filewriterapi-w3c-tests/improve

[webapi] Improve tests purpose description for File Writer API