[tracing] Improve and simplify startup tracing

This CL aligns behavior across child processes,
and supports tracing earlier during startup generally.
- Only Gpu process previously implemented early initialization, this
is now implemented on all child processes.
- Logic around SetupStartupTracingBlocking is simplified
as a result, we call it directly as soon as field trials
are initialized.

Gpu process on linux/chromeos restarts the tracing thread.
For simplicity, this isn't implemented in other processes and
we fallback to "later" startup tracing (needs_startup_tracing_after_sandbox_init), which is still earlier
than what we had before.

I encountered an underlying issue where UpdateDataSource
complains about missing DataSourceDescriptor.id;
I believe this started happening because the reordering
leads us to register e.g. v8 TrackEvent after we started tracing.
The consequences are benign; we don't really rely on DataSourceDescriptor other than being informative.
The fix is simply to forward the id through mojo.

Change-Id: I7f4afc5f97956bc7f97dc00741c317482be3ab5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6685790
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kramer Ge <fangzhoug@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Mikhail Khokhlov <khokhlov@google.com>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1482672}