Bug 925348: Spread out contact scroll rendering work.

This commit avoids spikes of DOM modifications during scrolling by tweaking
the tag_visibility_monitor parameters.  The new parameters result in more
frequent updates from the monitor which spreads the resulting work out
over time during scrolling.
Merge pull request #12778 from wanderview/contacts-render-jank

Bug 925348: Spread out contact scroll rendering work. r=arcturus