[webapi] Improve 3 cases for Selectors API Level2

Impacted tests(approved): new 0, delete 0, update 3
Unit test platform: [Android][IVI]
Unit test result summary: pass 3, fail 0, block 0
Merge pull request #998 from kangxu/kangxu/webapi/tct-selectorslevel2-w3c-tests/updateCases

[webapi] Improve 3 cases for Selectors API Level 2
[wrt][WEBQA-154] Add a file .gitkeep

- Build apk successfully, no error

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: Android
Android test result summary: Pass 1, Fail 0, Block 0
Merge pull request #1016 from wanghongjuan/wanghongjuan/wrt/WEBQA-154

[wrt][WEBQA-154] Add a file .gitkeep
[usecase-wrt][XWALK-2525][XWALK-2579]Add usecase for the feature

Impacted TCs num(approved): New 0, update 2, deleted 0
Unit test platform: [tizen]
Test Result: Pass 2, Fail 0
Merge pull request #1018 from linshen/shenlin/update/XWALK-2525

[usecase-wrt][XWALK-2525][XWALK-2579]Add usecase for the feature