See  issue 835197 

optimize-png-files.sh runs pngcrush, but does not take into account that a file is an APNG. So, for example, a png might not have transparency in frame 0, but need it for composing frames. The PNG is crushed by changing its color type to RGB (instead of RGB + alpha), resulting in an animation that does not look as intended.

The simplest solution would be to skip files that are APNGs. They can be identified by acTL chunks. But it looks like optimize-png-files.sh doesn't look at the data directly, and I didn't see (in a quick look) how to do any detection with pngcrush, so we may need to write more code to detect acTL.

Not sure who to assign this to. Added people from the original bug, plus agrieve@chromium.org who is an OWNER of the script's directory.