Update license allowlist

Adding generic 'Public Domain' for dependencies that only provide brief licenses like 'this is for use in public domain'. If the license classifier tool identifies a license, then that value should be used instead of this.

Change-Id: I42c9bc3eec7f91910645db3f8ae74d23d88e83da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6768430
Reviewed-by: Rachael Newitt <renewitt@google.com>
Commit-Queue: Rachael Newitt <renewitt@google.com>
Auto-Submit: Jordan Brown <rop@google.com>

metadata/fields/custom/license_allowlist.py

@@ -50,6 +50,7 @@ ALLOWED_SPDX_LICENSES = frozenset([
     "blessing",
     "CC0-1.0",
     "LicenseRef-Public-Domain-Gutenberg",
+    "LicenseRef-Public-Domain-SpanDSP",
     "LZMA-SDK-9.22",
     "Unlicense",
     # go/keep-sorted end
@@ -113,6 +114,7 @@ EXTENDED_LICENSE_CLASSIFIERS = frozenset([
     # go/keep-sorted start case=no
     "AhemFont",
     "LZMA",
+    "Public Domain",
     "public-domain-md5",
     "SPL-SQRT-FLOOR",
     # go/keep-sorted end