This CL adds tracing scopes for the various cppgc classes.
Scopes use TRACE_EVENT_BEGIN and TRACE_EVENT_END macros to report trace
events. To do so they need to include trace-event.h. For unified heap
builds, trace-event.h forwards to v8's src/tracing/trace-event.h. For
other builds, trace-event.h provides a subset of
src/tracing/trace-event.h that covers just the parts used by cppgc.

This CL covers what we need for traces and blink gc metrics (up to
renaming events from BlinkGC.* to CppGC.*). UMA and UKM are not yet
handled.

Bug: chromium:1056170
Change-Id: Id92e84b27259ff0aadae7692f3d79d30896fb8e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2540548
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71284}