[tracing] Cleanup TracingDelegate

- rename ContentBrowserClient::GetTracingDelegate to ContentBrowserClient::CreateTracingDelegate and return unique_ptr to avoid leaks and match what it does
- Remove TracingDelegate where it's not used
- Extract GenerateMetadataDict out of TracingDelegate since this can be
  done directly in TracingController.

Bug: 40257548
Change-Id: I735c8a420d8523a93e68a3ef51bcf2ac66ae4692
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398559


Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279965}