[webapi] Integrate 1 test for Animations

 - come from https://github.com/w3c/csswg-test/blob/master/css-animations-1/pseudoElement-002.html

Impacted tests(approved): new 1, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #1560 from yunxiaoxie/delta-cleanup-animations

[webapi] Integrate 1 test for Animations