Adding a new enum-typed Document Policy feature

When adding a new value_type: "Enum" feature to document_policy_features.json5, the following files must be updated:

1. document_policy_features.json5 — add the feature entry with value_type: "Enum" and a numeric default_value. The default value must use the underlying integer (not a named constant), since this file is processed before C++ enums are available.

2. document_policy_enum_values.h — add cases to both:

   • DocumentPolicyEnumTokenToValue() — maps header token strings to int32 values
   • DocumentPolicyEnumValueToToken() — maps int32 values back to token strings; return std::nullopt for any sentinel/default value that has no header representation

3. The enum header (e.g. js_profiling_mode.h) — define the C++ enum with explicit integer values matching those used in document_policy_features.json5.

4. document_policy_feature.mojom — add the feature to the DocumentPolicyFeature enum.

5. BUILD.gn / sources.gni — add any new header files to the appropriate build target.

Sentinel default values for enum features

The Document Policy framework always initialises every feature to its default_value, even when the header is absent. For enum features that should fall back to another policy when not explicitly set (rather than having a distinct "off" token), use 0 as a sentinel default and check value >= 1 in the consuming code. This keeps value 0 structurally unreachable from a header, making "not set" unambiguous.

See document_policy_enum_values.h and profiler_group.cc (GetProfilingMode) for a worked example with js-profiling-mode.