Follow the specification and validate tokens of the "x/w/h" descriptors
before applying the rules for parsing float-pointing numbers or
non-negative integers.
https://html.spec.whatwg.org/multipage/#parsing-a-srcset-attribute (step
13)
Testing: Improvements in the following tests
-
html/semantics/embedded-content/the-img-element/srcset/parse-a-srcset-attribute.html
Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>