perfetto: Only return unbound writers to the registry

Because we use StartupTraceWriterRegistries not only during startup, but
also for background tracing sessions, it's possible that one trace
writer is created during a former tracing session and later returned to
the startup tracing registry of another session. This could result in
the registry attempting to rebind an already bound writer, which causes
a nullptr crash.

Bug: 966307
Change-Id: I11a0ff43673b2fc35a5d48dcb89cd87d06e8ace1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626866
Auto-Submit: Eric Seckler <eseckler@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662716}
2 files changed