Silence another PNG print statement

src/images/SkImageDecoder_libpng.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
 #include "SkColor.h"
@@ skipping 110 matching lines @@
 #ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
 static int sk_read_user_chunk(png_structp png_ptr, png_unknown_chunkp chunk) {
     SkPngChunkReader* peeker = (SkPngChunkReader*)png_get_user_chunk_ptr(png_ptr);
     // readChunk() returning true means continue decoding
     return peeker->readChunk((const char*)chunk->name, chunk->data, chunk->size) ?
             1 : -1;
 }
 #endif
 
 static void sk_error_fn(png_structp png_ptr, png_const_charp msg) {
-    SkDEBUGF(("------ png error %s\n", msg));
+    if (!c_suppressPNGImageDecoderWarnings) {
+        SkDEBUGF(("------ png error %s\n", msg));
+    }
     longjmp(png_jmpbuf(png_ptr), 1);
 }
 
 static void skip_src_rows(png_structp png_ptr, uint8_t storage[], int count) {
     for (int i = 0; i < count; i++) {
         uint8_t* tmp = storage;
         png_read_rows(png_ptr, &tmp, png_bytepp_NULL, 1);
     }
 }
 
@@ skipping 868 matching lines @@
     return SkImageDecoder::kUnknown_Format;
 }
 
 SkImageEncoder* sk_libpng_efactory(SkImageEncoder::Type t) {
     return (SkImageEncoder::kPNG_Type == t) ? new SkPNGImageEncoder : nullptr;
 }
 
 static SkImageDecoder_DecodeReg gDReg(sk_libpng_dfactory);
 static SkImageDecoder_FormatReg gFormatReg(get_format_png);
 static SkImageEncoder_EncodeReg gEReg(sk_libpng_efactory);