[usecase-wrt][xwalk-3145] Add one test for pdf download

Impacted TCs num(approved): New 1, Update 0, Delete 0
Unit test Platform: Android
Android test result summary: Pass 0, Fail 1, Block 0
Merge pull request #1507 from wanghongjuan/usecase/xwalk-3145

[usecase-wrt][xwalk-3145] Add one test for pdf download