Bug 969779 - Use SpecialPowers to enable preference setting in oop. r=seth
* * *
1. Use |SpecialPowers.pushPrefEnv| to enable preference setting in oop.
2. Set the time of redrawing to be the time right after the image is discarded.
3. Modify the assertion to make the comparison non-trivial.
4. Remove <img>.
5. Move drawCanvas() on the image loadComplete.

--HG--
extra : amend_source : 2c9b4164ed5f10242cf4e7c4a784579706b91a21