Right now the Perfetto service runs on a dedicated
thread for a couple of reasons:
* The sequence needs to be locked to a specific thread, or Perfetto's
  thread-checker will barf.
* The PerfettoTracingCoordinator uses
  mojo::BlockingCopyFromString to pass the string to the tracing
  controller, which requires the WithBaseSyncPrimitives task trait and
  SingleThreadTaskRunners which use this trait need to be running on a
  dedicated trait to avoid blocking other sequences.
* If a client fills up its Shared Memory Buffer when writing a Perfetto
  event proto, it'll stall until the Perfetto service clears up space.
  This won't happen if the client and the service happens to run on the same
  thread (the Mojo calls will never be executed).

The above should be resolved before we move the Perfetto usage out from the
flag so we can run this on non-thread-bound sequence.