Add Tint contributors as owners for top-level build files

Adds rharrison@chromium.org and amaiorano@google.com as OWNERS for
some top-level build files that may be changed in Tint specific
changes to help ease reviews.

Change-Id: I32033b9a0aa3ac7aa7efaeffd39d6a652a905651
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/219054
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
diff --git a/OWNERS b/OWNERS
index d59b3c8..1e93f87 100644
--- a/OWNERS
+++ b/OWNERS
@@ -10,6 +10,12 @@
 per-file .gitignore=*
 per-file README.md=file://docs/OWNERS
 
+
+# Allows approving build rule changes, without giving owners for
+# everything
+per-file CMake*=rharrison@chromium.org,amaiorano@google.com
+per-file BUILD.*=rharrison@chromium.org,amaiorano@google.com
+
 # gitlinks that anyone can update just like DEPS
 per-file build=*
 per-file buildtools=*