Revert 121044 - 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.

Reverting due to compile errors on Linux:
http://chromegw.corp.google.com/i/chromium/builders/Linux%20Builder%20%28dbg%29%28shared%29/builds/17665/steps/compile/logs/stdio

BUG=112822

Review URL: http://codereview.chromium.org/9365007

TBR=cevans@chromium.org
Review URL: https://chromiumcodereview.appspot.com/9374001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@121090 0039d316-1c4b-4281-b951-d872f2087c98