[usecase] Add 1 usecase for Picture Element
Impacted tests(approved): new 1, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 0, fail 1, block 0
Comment: The image of source element can't be displayed normally
when the device width conform to the media strategy.
Merge pull request #1351 from kangxu/kangxu/usecase/usecase-webapi-xwalk-tests/new

[usecase] Add 1 usecase for Picture Element