Replace brucedawson@ in toolchain OWNERS file

I am the only owner in this file and I am leaving Google. This replaces
me with somebody else who is taking over toolchain updates.

Change-Id: Ief91deb0cb9a2b06bcdfc40e8b9711df5a33c570
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5906014
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Devon Loehr <dloehr@google.com>

win_toolchain/OWNERS

@@ -1 +1 @@
-brucedawson@chromium.org
+dloehr@google.com