Improve scroll restoration perf (bug 986625)

I've experimented with removing the min-height changes as well as the setTimeout and this looks to work both on Inari (B2G 1.2) and FF Android 28.0.1.  

I suspect the situation has been greatly improved by the landing of the deferred image loading branches so if that's the case I'm very glad we did that. :smiley_cat: 

This will need some really detailed QA to make sure across all devices we don't have any issues.