Ignore unknown descriptors in srcset.

The srcset spec have recently changed with regard to unknown descriptors,
for better future compatibility.
This CL aligns the implementation with the spec.

BUG=418919

Review URL: https://codereview.chromium.org/637343002

git-svn-id: svn://svn.chromium.org/blink/trunk@183487 bbb929c8-8fbe-4397-9dbb-9b2b20218538