Add SK_SUPPORT_LEGACY_PORTER_DUFF macro.

src/effects/SkPorterDuff.cpp

Index: src/effects/SkPorterDuff.cpp
diff --git a/src/effects/SkPorterDuff.cpp b/src/effects/SkPorterDuff.cpp
index 4cdcf798a1c527b869502ea44ffa163fb2621f16..5564ca7c0232439aadaba9e277c53554f54481ca 100644
--- a/src/effects/SkPorterDuff.cpp
+++ b/src/effects/SkPorterDuff.cpp
@@ -10,6 +10,8 @@
 #include "SkPorterDuff.h"
 #include "SkXfermode.h"
 
+#ifdef SK_SUPPORT_LEGACY_PORTER_DUFF
+
 /*  This file just exists as a compatibility layer, gluing the PorterDuff API
     into the (extended) SkXfermode API
  */
@@ -85,3 +87,5 @@ SkXfermodeProc SkPorterDuff::GetXfermodeProc(Mode mode) {
 SkXfermodeProc16 SkPorterDuff::GetXfermodeProc16(Mode mode, SkColor srcColor) {
     return SkXfermode::GetProc16(gPairs[mode].fXF, srcColor);
 }
+
+#endif