Support 'Refer to additional_readme_paths.json'

This is used by some README.chromium to reference transitive
dependencies, and is supported by licenses.py[0]

[0]
https://source.corp.google.com/h/chrome-internal/codesearch/chrome/src/+/main:tools/licenses/licenses.py;l=131-134

Bug: 398668341
Change-Id: Iac15b001614e7afa6eab981deaaf3d4aa5020d66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6296123
Auto-Submit: Jordan Brown <rop@google.com>
Commit-Queue: Jordan Brown <rop@google.com>
Reviewed-by: Rachael Newitt <renewitt@google.com>
Commit-Queue: Rachael Newitt <renewitt@google.com>

metadata/fields/custom/license_allowlist.py

@@ -202,6 +202,16 @@ WITH_PERMISSION_ONLY = frozenset([
     # go/keep-sorted end
 ])
 
-ALLOWED_LICENSES = ALLOWED_SPDX_LICENSES | EXTENDED_LICENSE_CLASSIFIERS
+# These are references to files that are not licenses, but are allowed to be
+# included in the LICENSE field.
+ALLOWED_REFERENCES = frozenset([
+    "Refer to additional_readme_paths.json",
+])
+
+ALLOWED_LICENSES = (
+    ALLOWED_SPDX_LICENSES
+    | EXTENDED_LICENSE_CLASSIFIERS
+    | ALLOWED_REFERENCES
+)
 ALLOWED_OPEN_SOURCE_LICENSES = ALLOWED_LICENSES | OPEN_SOURCE_SPDX_LICENSES
 ALL_LICENSES = ALLOWED_OPEN_SOURCE_LICENSES | WITH_PERMISSION_ONLY

metadata/tests/fields_test.py

@@ -123,6 +123,7 @@ class FieldValidationTest(unittest.TestCase):
                 "MIT",
                 "APSL-2.0, MIT",
                 "APSL-2.0 ,MIT",
+                "Refer to additional_readme_paths.json"
             ],
             error_values=[
                 "",
@@ -136,6 +137,7 @@ class FieldValidationTest(unittest.TestCase):
             warning_values=[
                 "Custom license",
                 "Custom, MIT",
+                "Refer to any_other_readme_paths.json",
             ],
         )