service: Remove some DCHECKs on writer {un,}registration.
service: Remove some DCHECKs on writer {un,}registration.

We may unregister a writer that was never registered (e.g. due to aborting startup tracing). Apparently chromium runs ASAN builds with
DCHECKs in the wild nowadays, so we're actually hitting this.

Either way, having DCHECKs on inputs to the tracing service doesn't
seem quite right anyway.

Bug: chromium:1067609
Change-Id: I9a5bbe2a6ad4667434f55f4b2c0e2e497c85a580

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto