Move libpng to //flutter/third_party/libpng

DEPS

@@ -636,7 +636,7 @@ deps = {
   'src/flutter/third_party/libjpeg-turbo/src':
    Var('flutter_git') + '/third_party/libjpeg-turbo' + '@' + '0fb821f3b2e570b2783a94ccd9a2fb1f4916ae9f',
 
-  'src/third_party/libpng':
+  'src/flutter/third_party/libpng':
    Var('flutter_git') + '/third_party/libpng' + '@' + '9187b6e12756317f6d44fc669ac11dfc262bd192',
 
   'src/flutter/third_party/libwebp':

build/secondary/third_party/libpng/BUILD.gn

@@ -0,0 +1,8 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+group("libpng") {
+  deps = [ "//flutter/third_party/libpng" ]
+  public_configs = [ "//flutter/third_party/libpng:libpng_public" ]
+}

skia/BUILD.gn

@@ -542,7 +542,7 @@ optional("png_decode") {
     "SK_CODEC_DECODES_ICO",
   ]
 
-  deps = [ "//third_party/libpng" ]
+  deps = [ "//flutter/third_party/libpng" ]
   sources = [
     "$_skia_root/src/codec/SkIcoCodec.cpp",
     "$_skia_root/src/codec/SkPngCodec.cpp",
@@ -553,7 +553,7 @@ optional("png_encode") {
   enabled = skia_use_libpng_encode && !skia_use_ndk_images
   public = skia_encode_png_public
 
-  deps = [ "//third_party/libpng" ]
+  deps = [ "//flutter/third_party/libpng" ]
   sources = skia_encode_png_srcs
 }