InputType::destroyShadowSubtree should AttachLazily

destroyShadowSubtree adds <shadow> elements to support the extra ShadowRoot
added by PasswordGeneratorButtonElement, we should lazy attach these. Eventually
we should just remove that extra shadow root hack though.

TBR=eseidel@chromium.org
BUG=276687

Review URL: https://chromiumcodereview.appspot.com/23201014

git-svn-id: svn://svn.chromium.org/blink/trunk@156412 bbb929c8-8fbe-4397-9dbb-9b2b20218538