Add block handling to the sizes parser

This CL fixes a bug where CSS blocks are not properly handled within the
sizes parser, resulting in invalid values considered as valid.

BUG=421000

Review URL: https://codereview.chromium.org/636843002

git-svn-id: svn://svn.chromium.org/blink/trunk@183380 bbb929c8-8fbe-4397-9dbb-9b2b20218538