Bug 959901 - [Calendar][Integration Test] "day_view_test.js" is permared r=gaye

this patch should fix the error but I'm not sure if error is caused by something that changed on gecko itself.

I could not replicate the bug locally but any build after https://travis-ci.org/mozilla-b2g/gaia/builds/16968000 fails (`scrollWidth` is always 1px wider than `clientWidth` or `getBoundingClientRect().width` based on my tests).