Specify restriction on file:// lines in Chromium OWNERS

Match the rule used in other Google open source projects described
at https://opensource.google/docs/thirdparty/documentation/#owners:
the first lines in the OWNERS file are

* Two committers, OR
* A file: directive pointing to another OWNERS file in //third_party

(Copied by jrn@ from an internal CL making the same change; fleshed
out the commit message.)

Change-Id: I4e031684941e90a5d9257d053ca0d36d7da6f67f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276813Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#784225}

docs/adding_to_third_party.md

@@ -92,7 +92,9 @@ See [Moving large files to Google Storage](https://goto.google.com/checking-in-l
 
 ### Add OWNERS
 
-Your OWNERS file must include 2 Chromium developer accounts. This will ensure
+Your OWNERS file must either list two Chromium developer accounts as the first
+two lines or include a `file:` directive to an OWNERS file within the
+`third_party` directory that itself conforms to this criterion. This will ensure
 accountability for maintenance of the code over time. While there isn't always
 an ideal or obvious set of people that should go in OWNERS, this is critical for
 first-line triage of any issues that crop up in the code.