Fix two srcset related bugs

Two problems with the current srcset parsing algorithm are fixed here:
1. When a set of descriptors contained only invalid descriptors, the resource wasn't ignored.
2. When more than 1 resource shared a single DPR value, the last one was taken into account, rather than the first one.

I've also added test and fixed current ones to prevent this from regressing in the future.

BUG=347998

Review URL: https://codereview.chromium.org/184523003

git-svn-id: svn://svn.chromium.org/blink/trunk@168241 bbb929c8-8fbe-4397-9dbb-9b2b20218538