client api: Select a tracing backing automatically
client api: Select a tracing backing automatically

Add a default argument for NewTrace() so the user doesn't need to
provide a tracing backend for the common case of only having one
registered backend.

Change-Id: Ie751851592d9371d995742e1e017b8c16422db9b

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