Fix zoom parameter regression

This patch fixes a regression caused by PR #3727.

__Steps to reproduce:__
Open http://mozilla.github.io/pdf.js/web/viewer.html#page=2&zoom=110.

__Expected result:__
The document should be scrolled such that the *top* of the second page is aligned with the top of the `viewerContainer`.

__Actual result:__
![zoom-parameter-regression](https://f.cloud.github.com/assets/2692120/1437136/1a8c6100-4168-11e3-93e4-bb5be6a7fa52.png)