rename some flattenable routines

Initialize felt a little too nebulous, and I think
the verb for "define registrar entry" is "register".

Change-Id: I52f2eb5df5acd46a8b38bb9ea9bb07f4ac8f3789
Reviewed-on: https://skia-review.googlesource.com/c/163990
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/include/effects/SkXfermodeImageFilter.h b/include/effects/SkXfermodeImageFilter.h
index c403d2f..8ccceb2 100644
--- a/include/effects/SkXfermodeImageFilter.h
+++ b/include/effects/SkXfermodeImageFilter.h
@@ -25,7 +25,7 @@
         return Make(mode, std::move(background), nullptr, nullptr);
     }
 
-    static void InitializeFlattenables();
+    static void RegisterFlattenables();
 
 private:
     SkXfermodeImageFilter();    // can't instantiate