defaultPassthroughServiceImplementation: default

Specify default argument to dPSI which allows the following calls:
- dPSI("foo", 1 /* maxThreads */);
- dPSI("foo");
- dPSI(1 /* maxThreads */);
- dPSI();

Test: compiles
Change-Id: If1f7100527583dd7d01c17c107282b043d710ee3
1 file changed