Use correct pseudo type for page selector specificity.

Page selector specificity did not work because the specificity calculations
in CSSSelector::specificityForPage() checked for PseudoClass instead of
PagePseudoClass.

BUG=294190

Review URL: https://chromiumcodereview.appspot.com/23597051

git-svn-id: svn://svn.chromium.org/blink/trunk@157973 bbb929c8-8fbe-4397-9dbb-9b2b20218538