ui: Only show the debug track if we have a valid engineId.
ui: Only show the debug track if we have a valid engineId.

The debug track needs to be associated with an engine(as the
othe tracks do). At the moment, it is possible to add a debug
track without having an engine, which leads to a crash.

Issue: https://github.com/google/perfetto/issues/307
Change-Id: I7acdd5efc4cf342e0ec90e718d5a9a0ef2123232
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto