Bug 1033403 - [Messages] We don't stop rendering if we tap "back" too so...

...on r=azasypkin

This moves the moment "initializeRendering" is called: in beforeEnter instead of
afterEnter.
good point. I can see argument for both sides:
* on one side we haven't read anything, so it makes sense to not mark it
* on another side, this can be a quick gesture to mark a thread as read (I think I do it quite often)

So I'd mark it as read.
What do you think?