Remove CachedUAStyle::hasAppearance.

As of f885f9b1178fa278beafd201fb92a93af2117d64, this variable is now
pointless, because the existence of a CachedUAStyle is only possible
when 'hasAppearance' is 'true'.

Review URL: https://codereview.chromium.org/639963005

git-svn-id: svn://svn.chromium.org/blink/trunk@183594 bbb929c8-8fbe-4397-9dbb-9b2b20218538