[webapi][xwalk-3039] Add missed icon of ViewportTest

- The wgt is missing icon.png when packing.
- Fail reason: XWALK-3055 viewport tag does not work on Tizen IVI

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0
Merge pull request #1350 from haoxli/webapi/xwalk-3039

[webapi][xwalk-3039] Add missed icon of ViewportTest