[CSS] Minor cleanups in CSS variables handling
https://bugs.webkit.org/show_bug.cgi?id=116318

Reviewed by Ryosuke Niwa.

No new tests, only a cleanup.

* css/CSSParser.cpp:
(WebCore::CSSParserString::substring): Optimize.
(WebCore::CSSParser::createPrimitiveVariableNameValue):
Remove intermediate variable.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150302 268f45cc-cd09-0410-ab3c-d52691b4dbfc