Enable float source size

The source-size in the sizes parsing algorithm was wrongfully returned as unsigned. This CL fixes that to enable floats there.

BUG=418905

Review URL: https://codereview.chromium.org/633643003

git-svn-id: svn://svn.chromium.org/blink/trunk@183401 bbb929c8-8fbe-4397-9dbb-9b2b20218538