Call AtomicString::number() in CSSGrammar.y

Call AtomicString::number() in CSSGrammar.y instead of String::number as
CSSSelector::setArgument() expects an AtomicString in argument. This
avoids uselessly creating a String and then convert it to an AtomicString.

R=eseidel
BUG=323739

Review URL: https://codereview.chromium.org/89843004

git-svn-id: svn://svn.chromium.org/blink/trunk@162787 bbb929c8-8fbe-4397-9dbb-9b2b20218538