cppgc_unittests uses tracing (e.g. when setting the TracingController in
tests.h) and thus needs to depend on v8_tracing. It cannot inherit the
dependency since the cppgc_unittests_sources v8_source_set doesn't
depend on cppgc.

Bug: v8:11203
Change-Id: I7227f696f841132f174435751a42a02ff865df26
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2563658
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71453}