libpng: update to 1.2.43

      I unpacked libpng 1.2.43 and did:
 cd third_party/libpng
 for x in $(ls -1) ; do cp /tmp/libpng-1.2.43/$x .; done

The diffs appear to all be safe. The Chromium specific config is in pngusr.h
and isn't touched by this change.

1.2.43 fixes http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-0205

c7013d773eb45b8ed6a864e419e544106a0dd95d  /tmp/libpng-1.2.43.tar.xz

BUG=38512
TEST=none

http://codereview.chromium.org/1118002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@41972 0039d316-1c4b-4281-b951-d872f2087c98