Hi gjt211,
I know this is an old post, but here is what I found (sorry to the original/source of this as I am not sure where I originally found it but thank you!).
You found it 3 days ago on the Arduino Zero forum: WDT Watchdog timer code - Arduino Zero - Arduino Forum.
The orginal source is here: Found and fixed a bug in sketch for Arduino Zero | Details | Hackaday.io.
By the way, there's no need to use the WDT_CONFIG_PER_X definitions, as the SAMD21 CMSIS register definitions have changed in version 1.2.0.
Just replace the line:
REG_WDT_CONFIG = WDT_CONFIG_PER_7; // Set the WDT reset timeout to 1 second
with:
REG_WDT_CONFIG = WDT_CONFIG_PER_1K; // Set the WDT reset timeout to 1 second