Move onBindViewHolder from NewTabPageViewHolder to NewTabPageItem.
This moves NewTabPageItem further away from being a generic base class for holding the data shown in the RecyclerView, and will allow pushing the responsibility to update the view into the merged item/group interface.
BUG=
616090
Committed:
https://crrev.com/42d87c5d7b85ba9d6df6bcaa8b1f7830e5e8430f
Cr-Commit-Position: refs/heads/master@{#417291}