This patch adds EmbedderGraph::Node::NamePrefix method that will be used
by Chrome for detached DOM nodes.

Bug: chromium:811925
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I89d3b88a3b90ed85addb1d34f08dd15e0559aa9a
Reviewed-on: https://chromium-review.googlesource.com/926362
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51464}