Blacklist Android Chrome for webp conversion

[GoogleCodeExporter]

Visit the following url from Chrome on Android:

http://www.modpagespeed.com/rewrite_images.html?ModPagespeed=on&ModPagespeedFilt
ers=rewrite_images,inline_images,resize_images,insert_image_dimensions,convert_j
peg_to_webp

Note that the puzzle image is blue.  This appears to be a bug in webp handling 
on Android Chrome.  We're going to need to blacklist versions of Chrome on 
Android that have this bug.

Until we have a fix, disabling convert_jpeg_to_webp will work around the issue.

Original issue reported on code.google.com by jmaes...@google.com on 24 Jul 2012 at 5:09