While it is undeniable that the OSX keychain adds a roadblock to the theft, many average users would happily enter their password if the box was displayed when they ran up their browser (even if the browser wasn't the originating process) and likely also fall for a fake keychain prompt.
I think the keychain is a good thing (just as it is in Android). Just wanted to make the point that the keychain for your average non-power user is a minor roadblock in theft, rather than a "real" security feature.
At least Chrome extends the supplied OS security features and doesn't try to re-engineer them from scratch. This makes me more comfortable with Chrome rather than less.
I'm not really sure there is really any defense yet devised for phishing attacks against 'average non-power users.'
I'm not disputing you though, I think it is a problem. But unrelated to how securely a given app stores sensitive info, which is what OP is about.
Firefox, out of the box, does _not_ use the OSX keychain even on OSX. Sadly. It ought to.