Fixes an animation glitch caused by an incorrect editor size.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/363).

This PR should also remove any chance that [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/301) could happen again.  Unfortunately I could not reproduce this latter issue even before my changes, but I believe it has to do with some recent versioning issues we had between WPiOS 4.6 and 4.7.

/cc @bummytime