Support user-configurable function to create IDWriteFactory

include/ports/SkTypeface_win.h

Index: include/ports/SkTypeface_win.h
diff --git a/include/ports/SkTypeface_win.h b/include/ports/SkTypeface_win.h
index 77173e2cf7ca7853c49682b70ff9dbda58de7c32..bbef366ae703404f5b6e1493660e4b8d334929b1 100644
--- a/include/ports/SkTypeface_win.h
+++ b/include/ports/SkTypeface_win.h
@@ -33,6 +33,12 @@ SK_API void SkLOGFONTFromTypeface(const SkTypeface* typeface, LOGFONT* lf);
   */
 SK_API void SkTypeface_SetEnsureLOGFONTAccessibleProc(void (*)(const LOGFONT&));
 
+/**
+ * Set an optional callback to allow the user to create the IDWriteFactory.
+ */
+struct IDWriteFactory;
+SK_API void SkSetDirectWriteFactoryProc(void (*)(IDWriteFactory**));
+
 // Experimental!
 //
 class SkFontMgr;