Fixes an animation glitch caused by an incorrect editor size.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/363).
Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/394).
Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/301).

/cc @bummytime