Add console errors and usecounter when srcset candidates are dropped

Debugging markup is hard. 
I'm adding console logs here so that it'd be easier for authors to spot that they have invalid srcset descriptors,
and that their candidate resources are being dropped.

I also added a UseCounter, so we'd have a better idea on how often authors get their descriptors wrong.

BUG=422049

Review URL: https://codereview.chromium.org/649183007

git-svn-id: svn://svn.chromium.org/blink/trunk@184069 bbb929c8-8fbe-4397-9dbb-9b2b20218538