Don't use system libpng by default. It causes security maintenance problems for ChromeOS. It's worth noting that we also don't use the system library for many other important components such as libxml.
Don't use system libpng by default. It causes security maintenance problems
for ChromeOS. It's worth noting that we also don't use the system library
for many other important components such as libxml.

BUG=112822
R=wad,abarth
Review URL: https://chromiumcodereview.appspot.com/9365007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@121199 0039d316-1c4b-4281-b951-d872f2087c98

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