Update libpng to 1.6.22

third_party/libpng/README.chromium

Index: third_party/libpng/README.chromium
diff --git a/third_party/libpng/README.chromium b/third_party/libpng/README.chromium
index e767a00e4bb5d831d9571900839d29d74e182a1b..3263906f3c1d15e098deb296e45be1ba3379b3d0 100644
--- a/third_party/libpng/README.chromium
+++ b/third_party/libpng/README.chromium
@@ -1,17 +1,23 @@
 Name: libpng
 URL: http://libpng.org/
-Version: 1.2.56
+Version: 1.6.22
 Security Critical: yes
 License: libpng license
 License Android Compatible: yes
 
 Description:
-Updated to 1.2.56, no changes to the source files but all unneeded files
-stripped.
+Updated to 1.6.22, stripped all unneeded files.
 
-Our custom configuration options are defined in pngusr.h. This was previously
-called mozpngconf.h, which was copied from Mozilla and modified by Apple (hence
-the wk_* names):
-- Unset PNG_NO_READ_PACK on Android in pngusr.h, required by freetype to support
-  color glyphs.
-- Configure PNG chunk user limits (crbug.com/117369)
+- Turn on SSE optimizations for Intel platforms by running:
+  "patch -i contrib/intel/intel_sse.patch -p1"
+- Use custom configuration file pnglibconf.h, which turns off all features that
+  are not in use.
+- Use custom prefix file pngprefix.h, which avoids namespace conflicts with
+  pdfium's copy of libpng.
+- Configures custom png chunk user limits (crbug.com/117369) in pnglibconf.h.
+- Fix for potential memory leak from upstream:
+  https://github.com/glennrp/libpng/commit/8d1672537324d99c6e65eb78fc1678cdd46f75bd
+  https://github.com/glennrp/libpng/commit/6c7459e455a4f7d95c7700e9326bb12396e6eb1f
+- Fix for handling empty first IDAT chunk from upstream:
+  https://github.com/glennrp/libpng/commit/3f46c67c6989f480bd932428aa1705f6625dbabf
+  https://github.com/glennrp/libpng/commit/81f0273d54aa9de663253b197b3c8228d659cc36