clean up flattenable macro cruft

I'm finding it easier to reason about the code without most of these
macros, but SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY still looks useful.

Adding flattenable:: to CreateProc helps enforce that we pass the
correct type name.  The name is not currently used, but I'd like to.

Change-Id: I87c77eb6020fb3ae4696d8f73d09de62fd814875
Reviewed-on: https://skia-review.googlesource.com/c/163840
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
24 files changed