[cobalt] Change visibility to use $cobalt_root

Change-Id: I74ae06517cac0c4a28cfe4bf660f65544dcc4581
diff --git a/BUILD.gn b/BUILD.gn
index d0c7c93..66ff370 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,7 +7,7 @@
 # moves out-of-tree.
 visibility = [
   "//third_party/abseil-cpp/*",
-  "//third_party/cobalt/*",
+  "$cobalt_root/*",
   "//third_party/tink/*",
 ]
 
diff --git a/absl/synchronization/BUILD.gn b/absl/synchronization/BUILD.gn
index 58e860e..1846868 100644
--- a/absl/synchronization/BUILD.gn
+++ b/absl/synchronization/BUILD.gn
@@ -7,7 +7,7 @@
 # moves out-of-tree.
 visibility = [
   "//third_party/abseil-cpp/*",
-  "//third_party/cobalt/*",
+  "$cobalt_root/*",
   "//third_party/tink/*",
 ]