Tracing: Lower priority of TracedProcess interface bind requests

Currently the tracing service will unconditionally connect to
the TracedProcess interface on all services, which is a feature
provided by the service_manager client lib. This change allows
RequestQuit() calls services make, to still succeed even if
there's an outstanding TracedProcess bind request, as it's
not the service itself which handles this request.

See discussion in https://chromium-review.googlesource.com/c/chromium/src/+/1452767

R=ssid@chromium.org

Change-Id: Id7a4a6dc8eaa982b0ce24d7a699b430b3c07f833
Reviewed-on: https://chromium-review.googlesource.com/c/1457214


Commit-Queue: oysteine <oysteine@chromium.org>
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: ssid <ssid@chromium.org>
Auto-Submit: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629865}