manifest: Use chromiumos-overlay from third_party.

Currently, R77 and R81 uses chromiumos-overlay from
https://cos.googlesource.com/mirrors/cros/chromiumos/overlays/chromiumos-overlay
where R85 and above uses from
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/

This creates if-else cases in the release notes tool while generating
changelog for chromiumos-overlay as the repo names are different.
As the commits currently used for R77 and R81 are present in
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/,
change the repo to
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/
so that all the branches are using the same repo and it avoids the
if-else condition for generating changelog.

BUG=b/172264418
TEST=repo init and sync

Change-Id: Iab8581a24424980959daa70a365f36420412deaf
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/8281
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
diff --git a/external_full.xml b/external_full.xml
index 946b1e9..e5defbc 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -47,7 +47,7 @@
            revision="67862b2d1c5ec9ad8a5b0a5471166ae80e342fd8" />
   <project path="src/third_party/chromiumos-overlay"
            sync-c="true"
-           name="mirrors/cros/chromiumos/overlays/chromiumos-overlay"
+           name="third_party/overlays/chromiumos-overlay"
            revision="50547eafb48c5f520e598dbdf93c8f01ac4a49a2" />
   <project path="src/third_party/eclass-overlay"
            name="mirrors/cros/chromiumos/overlays/eclass-overlay"