[misc][XWALK-3000] Add usecase for File API PERSISTENT

Impacted tests(approved): new 1, update 0, delete 0
Unit test platform: [IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #1460 from yunxiaoxie/usecase-fileapipersistent

[misc][XWALK-3000] Add usecase for File API PERSISTENT