HeapVisitor is similar to StaticVisitor but uses virtual dispatch
instead of static function table. It is intended as replacement
of StaticVisitor using the CRTP.

This CL also changes the concurrent marker to use the HeapVisitor.

BUG=chromium:709075

Review-Url: https://codereview.chromium.org/2808093003
Cr-Commit-Position: refs/heads/master@{#44948}