PseudoElement::setNeedsStyleRecalc() should compute a new style

We need to clear the cached pseudo style whenever a PseudoElement specifically
requests a new style otherwise PseudoElement::customStyleForRenderer() will just
get the cached (old) style off the parent.

BUG=314968

Review URL: https://codereview.chromium.org/59013002

git-svn-id: svn://svn.chromium.org/blink/trunk@161314 bbb929c8-8fbe-4397-9dbb-9b2b20218538