Add magchen@ as an owner of GpuWatchdogThread in gpu/ipc/service

Change-Id: I54e6dc2320a3be43f5166244be0740990b991b4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545723Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Maggie Chen <magchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828408}

gpu/ipc/service/OWNERS

 # Mac stuff.
 per-file *_mac*=ccameron@chromium.org
+
+# Gpu Watchdog
+per-file *gpu_watchdog_thread*=magchen@chromium.org
\ No newline at end of file