This CL handles the chromium subdir, but not google_chrome.

pngcrush v1.7.15 removes txt/xml/time chunks with switches:
  -reduce -brute -rem iTXt -rem tEXt -rem zTXt -rem tIME
pngslim v1.0.1 runs a battery of compression operations.

The size difference reported uploading Patch Set 2 is:
Before: 1,293,629 bytes
After:    966,999 bytes
Delta:    326,630 bytes

Rietveld diff and try job patches fail, local build is fine.

BUG=76282
TEST=same images, less bytes.

Review URL: http://codereview.chromium.org/7782022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@100218 0039d316-1c4b-4281-b951-d872f2087c98