Introduce TextLinkColors.

Unburden Document from the knowledge about Color by splitting off all
Color-carrying members into their own class. Also straighten up layering
by moving text color logic out of RenderBox.

R=eae, eseidel
BUG=237268

Review URL: https://chromiumcodereview.appspot.com/18802002

git-svn-id: svn://svn.chromium.org/blink/trunk@153739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
HTMLKeygenElement should use didAddUserAgentShadowRoot and not create subclasses of HTMLSelectElement

HTMLKeygenElement was manually managing the UserAgent ShadowRoot instead of using
the "framework" with didAddUserAgentShadowRoot. It also created a subclass of
HTMLSelectElement just to set the pseudoId on it to "-webkit-keygen-select". We
can remove that and then make HTMLSelectElement FINAL.

Review URL: https://chromiumcodereview.appspot.com/18809002

git-svn-id: svn://svn.chromium.org/blink/trunk@153740 bbb929c8-8fbe-4397-9dbb-9b2b20218538