Refactor PointerUpdatingVisitor.

This patch extracts typed slot processing logic from
the PointerUpdatingVisitor to a set of functions.

Each of these functions takes an untyped slot processing function
and applies it to a typed slot.

BUG=chromium:612847
LOG=NO

Review-Url: https://codereview.chromium.org/1991843003
Cr-Commit-Position: refs/heads/master@{#36357}