Add third_party/depot_tools to OWNERS for roller

Change-Id: I2892498fc5a80184654cf9b5476ca8ec76b69d08
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/178440
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
diff --git a/OWNERS b/OWNERS
index 8dbe33b..2909d4d 100644
--- a/OWNERS
+++ b/OWNERS
@@ -19,6 +19,7 @@
 per-file third_party/angle=*
 per-file third_party/catapult=*
 per-file third_party/clang-format/script=*
+per-file third_party/depot_tools=*
 per-file third_party/dxc=*
 per-file third_party/dxheaders=*
 per-file third_party/glfw=*