Add 3p repo managers to some upstream code reviews (youtube#2284)

b/322206267

.github/CODEOWNERS

@@ -11,10 +11,10 @@
 /tools/metrics/ @joeltine
 
 # Temporary monitoring for merges to Chromium upstreams
-/base/ @andrewsavage1
-/build/ @andrewsavage1
-/crypto/ @andrewsavage1
-/net/ @andrewsavage1
-/third_party/abseil-cpp/ @andrewsavage1
-/third_party/modp_b64/ @andrewsavage1
-/url/ @andrewsavage1
+/base/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/build/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/crypto/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/net/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/third_party/abseil-cpp/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/third_party/modp_b64/ @andrewsavage1 @youtube/cobalt-3p-repository-owners
+/url/ @andrewsavage1 @youtube/cobalt-3p-repository-owners