Support author Shadow DOM for INPUT elements.

When the first author ShadowRoot is added to an input element, we need to
disable all of UI-specific behavior.

 * Add Element::didAddShadowRoot callback, and HTMLInputElmeent::
  didAddShadowRoot replaces m_inputTypeView with one which disables
  UI-specific behavior, and re-attach a renderer.

 * InputType::destroyShadowSubTree: We should not remove the content of
  author ShadowRoots.

We don't need to take care of removal of author ShadowRoots.  It never
happens.

BUG=274205

Review URL: https://chromiumcodereview.appspot.com/21165005

git-svn-id: svn://svn.chromium.org/blink/trunk@157510 bbb929c8-8fbe-4397-9dbb-9b2b20218538