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

include/core/SkImageEncoder.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageEncoder_DEFINED
 #define SkImageEncoder_DEFINED
 
@@ skipping 88 matching lines @@
     SkImageEncoder* Create##codec() { return new Sk##codec; }
 
 // All the encoders known by Skia. Note that, depending on the compiler settings,
 // not all of these will be available
 DECLARE_ENCODER_CREATOR(JPEGImageEncoder);
 DECLARE_ENCODER_CREATOR(PNGImageEncoder);
 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
 // This has to be defined outside SkImageEncoder. :(
 typedef SkTRegistry<SkImageEncoder*(*)(SkImageEncoder::Type)> SkImageEncoder_EncodeReg;
 #endif