Verify srcset descriptors are valid ints/floats

According to the spec, srcset descriptors should be valid
non-negative integers,and valid floating point numbers.
This CL makes it so that when they are not, the descriptor is dumped.

BUG=418919

Review URL: https://codereview.chromium.org/641693002

git-svn-id: svn://svn.chromium.org/blink/trunk@183446 bbb929c8-8fbe-4397-9dbb-9b2b20218538