Make applyAuthorStyles() and resetStyleInheritance() non-virtual

ShadowRoot::resetStyleInheritance() never needed to be virtual since we only call it
on ShadowRoot intances and there are no subclasses. applyAuthorStyles() only makes
sense on ShadowRoot and having the virtual call for every element during recalcStyle
is super sad. Instead we just check if the root is a ShadowRoot and directly call it.

Review URL: https://chromiumcodereview.appspot.com/22451004

git-svn-id: svn://svn.chromium.org/blink/trunk@155852 bbb929c8-8fbe-4397-9dbb-9b2b20218538