Boost pages based on their namespace

We take a reasonable amount of care to do this as efficiently as possible:
1.  Just skip it if all namespaces get the same boost.
2.  Never boost any namespace when the boost would be a noop.
3.  Combine all the namespaces that get the same boost into one filter.

Bug: 61053
Change-Id: Ica89a3c6696ba761bae4643238885db7fc507142
Merge "Boost pages based on their namespace"