Add test for platform encoders, turn off platform encoders by default

include/core/SkImageEncoder.h

Index: include/core/SkImageEncoder.h
diff --git a/include/core/SkImageEncoder.h b/include/core/SkImageEncoder.h
index 706d0c7bb2a1e5efc4591102a352d1fac70aab9d..8049089b90e43a6a1897a098991010d14b8b6889 100644
--- a/include/core/SkImageEncoder.h
+++ b/include/core/SkImageEncoder.h
@@ -106,11 +106,11 @@ DECLARE_ENCODER_CREATOR(KTXImageEncoder);
 DECLARE_ENCODER_CREATOR(WEBPImageEncoder);
 
 #if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
-DECLARE_ENCODER_CREATOR(PNGImageEncoder_CG);
+SkImageEncoder* CreateImageEncoder_CG(SkImageEncoder::Type type);
 #endif
 
 #if defined(SK_BUILD_FOR_WIN)
-DECLARE_ENCODER_CREATOR(ImageEncoder_WIC);
+SkImageEncoder* CreateImageEncoder_WIC(SkImageEncoder::Type type);
 #endif
 
 // Typedef to make registering encoder callback easier