This is a follow-up to https://chromium-review.googlesource.com/904164
now that I've realized raw TRACE_EVENTs work in v8 (initially thought
had to go through heap()->tracer()).

After adding events to EvacuatePage(), the subitem event for evacuation tasks
became redundant. Since this results in a better output this CL applies
this approach to all paralell work items.

Not using TRACE_BACKGROUND_GC also avoids polluting logs with subitems
when using v8's tracing flags.

R=mlippautz@chromium.org

Bug: chromium:651354
Change-Id: I62efbde69da2a0a9bd255e34093d64924e339974
Reviewed-on: https://chromium-review.googlesource.com/934263
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51544}