[usecase-webapi] Fix bugs of Canvas.

- Clean the show frame when click the buttons everytime.
- Better to enhance the test: Roate 90, 180, 270, 360. Scale 50%, 200%.
- Make sure that each action based on the original picture.

Impacted tests(approved): New 0, Update 1, Delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: Pass 1, Fail 0, Block 0
Merge pull request #545 from minxhuang/hm/misc/usecase-w3c/Canvas-hotfix

[usecase-webapi] Improve Canvas usecase.