Rename SkCodec_libpng to SkPngCodec

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1613043003

Review URL: https://codereview.chromium.org/1613043003

gyp/codec.gyp

@@ -41,7 +41,6 @@
         '../src/codec/SkBmpRLECodec.cpp',
         '../src/codec/SkBmpStandardCodec.cpp',
         '../src/codec/SkCodec.cpp',
-        '../src/codec/SkCodec_libpng.cpp',
         '../src/codec/SkGifCodec.cpp',
         '../src/codec/SkIcoCodec.cpp',
         '../src/codec/SkJpegCodec.cpp',
@@ -49,6 +48,7 @@
         '../src/codec/SkJpegUtility_codec.cpp',
         '../src/codec/SkMaskSwizzler.cpp',
         '../src/codec/SkMasks.cpp',
+        '../src/codec/SkPngCodec.cpp',
         '../src/codec/SkSampler.cpp',
         '../src/codec/SkSampledCodec.cpp',
         '../src/codec/SkSwizzler.cpp',

src/codec/SkCodec.cpp

@@ -7,7 +7,6 @@
 
 #include "SkBmpCodec.h"
 #include "SkCodec.h"
-#include "SkCodec_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkData.h"
 #include "SkGifCodec.h"
@@ -15,6 +14,7 @@
 #if !defined(GOOGLE3)
 #include "SkJpegCodec.h"
 #endif
+#include "SkPngCodec.h"
 #include "SkStream.h"
 #include "SkWbmpCodec.h"
 #include "SkWebpCodec.h"

src/codec/SkIcoCodec.cpp

@@ -6,11 +6,11 @@
  */
 
 #include "SkBmpCodec.h"
-#include "SkCodec_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkData.h"
 #include "SkIcoCodec.h"
+#include "SkPngCodec.h"
 #include "SkStream.h"
 #include "SkTDArray.h"
 #include "SkTSort.h"

src/codec/SkPngCodec.cpp

@@ -5,12 +5,12 @@
  * found in the LICENSE file.
  */
 
-#include "SkCodec_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
 #include "SkBitmap.h"
 #include "SkMath.h"
+#include "SkPngCodec.h"
 #include "SkSize.h"
 #include "SkStream.h"
 #include "SkSwizzler.h"