[oilpan] Move name callbacks into GCInfo

This moves the NameCallback from the fast path (trace descriptor) to a
slow path in GCInfo. It also allows for implementing naming for the rest
of the DOM.

Bug: chromium:841830
Change-Id: I61e8043855eb673ea173b92627e78df3e7eff0ab
Reviewed-on: https://chromium-review.googlesource.com/1069964
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561040}