This patch adds a new interface called RootVisitor and changes the root
iteration functions to accept a RootVisitor instead of an ObjectVisitor.

Future CLs will change ObjectVisitor to provide the host object to all
visiting functions, which will bring it in sync with static visitors.

Having separate visitors for roots and objects removes ambiguity in
VisitPointers and reduces chances of forgetting to record slots.

This is intended as pure refactoring. All places that require behavior
change are marked with TODO and will addressed in future CLs.

BUG=chromium:709075

Review-Url: https://codereview.chromium.org/2801073006
Cr-Commit-Position: refs/heads/master@{#44852}