Enable a srcset 0x descriptor

According to recent spec changes, a "0x" descriptor should be correctly parsed,
and imply intrinsic dimensions of infinity to the image.
This CL enables that, and tests that intrinsic dimensions are impacted accordingly.

BUG=378252

Review URL: https://codereview.chromium.org/305643006

git-svn-id: svn://svn.chromium.org/blink/trunk@175196 bbb929c8-8fbe-4397-9dbb-9b2b20218538