- Re-compresses all the image data, average savings 1%-4% in filesize.
- Removes the "Created with Adobe Whatever" string from all PNGs, shrinking
  the files marginally even when the image data was already optimal.

This shaves more than 100k off of chrome.pak.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/2806029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50754 0039d316-1c4b-4281-b951-d872f2087c98