Enable flattening/unflattening with custom unflatten procs

Enable flattening/unflattening with custom unflatten procs

Now flattenables are serialized using a string name, so that
flattenables do not necessarily need to be registered before
serialization.  They just need to override getTypeName().

Allows custom unflatten procs to be set on the SkReadBuffer.
This is optional if the flattenable is registered, but otherwise
must be called.

This was split off from:
https://codereview.chromium.org/1837913003/

BUG=skia:
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Review URL: https://codereview.chromium.org/1858323002