The marking speed is computed from incremental marking steps reported.
In case of embedder tracing, such steps should not include time spent
processing embedder memory.

Bug: chromium:1044630, chromium:1049084
Change-Id: Ie7b5af569b4114dd8dffd2d85eaf28ece9dfa18c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062394
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66316}