I am not sure, what the question is, but i know what did you forget. You have also to protect the gate driver. This is because between the gate and the drain (and source) there is some capacitance which in case of ESD discharge will just conduct, so a voltage surge is possible on the gate.
upd. in case of such surge one of the implications may be opening the valve. not very nice.
The first thing that comes to mind is that ESD testing should be done on points that are normally accessible during normal operation. Since you haven't described what you are building, in your question it makes up for kind of a vague answer. The testing is also not described in your question, and the most important thing is what points the ESD gun is connected to. The human body model is with respect to earth ground. 
When you say positive strike, do you mean positive with respect to earth ground? Positive with respect to what? The current is going to take the lowest impedance path back to the source. You have to connect the negative or ground side of the ESD gun to something. Normally it is connected to earth ground or chassis ground. The current must travel back to the source, so if you hit the board with the ESD gun connected to ground, it has several 'potential' paths that it could take back. There is the usb shield, the usb ground, the D+ and D- wires, Vbus and the air. The Air is like a 10^6 to 10^9 resistor with a few pF's of capacitance. The D+ and D- wires have at least the resistance of Rt plus whatever is on the driving end of the cable. Vbus would probably have a voltage regulator on it so it would be less than that of Rt. The ground has the resistance of the cable and the connectors, which is probably the lowest of all, being an ohm or two to earth ground (since ground and earth ground are connected somewhere down the line).
So the answer is, its going to take the earth ground back to the source (if you have the esd gun connected to ground, and all paths will see it to some extent, the ground will take most of the current)
Back to the design:
The whole reason for having a shield around USB is to shunt noise and ESD away from the grounding wire that is next to the V+ and V- cables. Because of mutual inductive coupling, you could get some transients that cross over to the datalines and at a minimum create noise that knocks out the packet that was sent.
The best thing would be to use the shield to shunt noise and ESD away from the datalines and usb ground. The next best thing (and not recommended) would be to shunt the ESD down the ground wire of the usb cable.
If the pcb is being handled by humans try and design the pcb so ground is the first thing seen by an ESD event. The ideal situation would be to have a metal case that encloses your device tied to the usb shield. If you can't tie the enclosure to the shield (not recommended) tie it to ground. If you cant have an enclosure then try and use guard rings and a nice healthy ground plane.
Best Answer
The idea of ESD circuits is you want the current from the ESD spike to be shunted away from sensitive electronics. Typically this is done by connecting the diodes to a chassis ground or shield, so the pathway will not be through the ground but out through another pathway and back to the source (usually earth ground which is 0V). If your design doesn't have a chassis ground, then shunt it to the next best place, ask yourself, where do I want these currents to go once they have passed through the diode. With careful PCB planning (like locating a ground connector close to the TVS diodes) can direct currents away from the electronics.
That would be one option, one way I've done it is have a separate chassis ground for the shields of connectors and protected inputs, which is connected via fastener to chassis ground.