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Add support for a new property on function types, serializableFunction.

IDL Example:
callback SerializableFunction = void();
dictionary SerializableFunctionObject {
  [serializableFunction]SerializableFunction func;
};

JSON Example:
{
  "type": "object",
  "id": "SerializableFunctionObject",
  "properties": {
    "func": {
      "name": "func",
      "serializableFunction": true,
      "type": "function",
      "parameters": []
    }
  }
}

If this trait is present, functions will be serialized and deserialized
as strings, rather than as empty base::DictionaryValues.

Add testing for the same.

Note: this only provides support in the JSON schema compiler (which
includes support in our auto-generated strong-typing code); a follow-up
will add support in our bindings code for parsing the values passed
from extensions.

Bug: 1144841
Change-Id: I430fb211166a5ccffaa6b478642e98115fd25565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518717
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825727}