Revert of Delete SkFlattenable::Type (patchset #2 id:20001 of https://codereview.chromium.org/1834303003/ )

Reason for revert:
Used by Chrome.

c:\b\build\slave\workdir\build\src\cc\playback\compositing_display_item.cc(53): error C2039: 'GetFlattenableType': is not a member of 'SkColorFilter'
c:\b\build\slave\workdir\build\src\third_party\skia\include\core\skshader.h(19): note: see declaration of 'SkColorFilter'
c:\b\build\slave\workdir\build\src\cc\playback\compositing_display_item.cc(53): error C3861: 'GetFlattenableType': identifier not found

Original issue's description:
> Delete SkFlattenable::Type
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1834303003
>
> Committed: https://skia.googlesource.com/skia/+/99d9231f6a4cb6b85b8637e9d8ae32f8bd7c466f

TBR=reed@google.com,msarett@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1853383002
diff --git a/tests/SerializationTest.cpp b/tests/SerializationTest.cpp
index b590fb5..15b7382 100644
--- a/tests/SerializationTest.cpp
+++ b/tests/SerializationTest.cpp
@@ -42,7 +42,7 @@
         writer.writeFlattenable(flattenable);
     }
     static void Read(SkValidatingReadBuffer& reader, T** flattenable) {
-        *flattenable = (T*)reader.readFlattenable();
+        *flattenable = (T*)reader.readFlattenable(T::GetFlattenableType());
     }
 };