Removing tEXt, iTXt, tIME, mk*, prVW, and vpAg chunks saves over 2 MB. The biggest savings is from iTXt chunks containing XML, seemingly left by Adobe ImageReady, which totaled 1.6 MB in 1600 files.

Also update chrome/app/theme/README with extra pngcrush flags to drop the worst offenders.

Chunks I left in place but that we may not need: cHRM, gAMA, iCCP, pHYs, sBIT.

TBR=ben@chromium.org
BUG=146632
TEST=pngtopnm|md5sum unchanged for all affected images

Review URL: https://codereview.chromium.org/10916327

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@157065 0039d316-1c4b-4281-b951-d872f2087c98