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This version mostly just directs the TracingController using the specified BackgroundTracingConfig and pushes the compressed trace out to the BackgroundTracingUploadSink.

Specifically, we implement the PREEMPTIVE_TRACING_MODE for the rule MONITOR_AND_DUMP_WHEN_TRIGGER_NAMED, which should allow us to get an experiment going on desktop with a simple trigger and upload.

We can then follow up with additional CL's implementing the rest of the functionality from the clientside doc below.

Needs to land first: https://codereview.chromium.org/1088673003/

Slow Reports Clientside: https://docs.google.com/document/d/1qZmXmodxOKmsTRO27z2WlH2h9Kpf-kjV-k-1pJIogIE/edit?pli=1

go/slow-reports

Review URL: https://codereview.chromium.org/1089253003

Cr-Commit-Position: refs/heads/master@{#330942}