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Add support for serializing JS functions to strings, rather than to
empty base::DictionaryValues. This will only be done for functions that
are marked with the trait serializableFunction in the schema.

Add testing for the same.

Bug: 1144841
Change-Id: I3990447ff336143a38eb97bf91a90503b0ee5c3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518451
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825765}