Access a password generator element with its ID.

This reduces the size of PasswordInputType objects, and simplifies the code.
Also, this CL removes a stale comment in PasswordGeneratorButtonElement.cpp. We
have a layout test now.

- PasswordGeneratorButtonElement::create() adds its ID.
- Remove InputType::passwordGeneratorElement().
  HTMLInputElement::passwordGeneratorElement() search UA shadow tree
  for it.
- Remove PasswordInputType::m_generatorButton
- Remove PasswordInputType::destroyShadowSubtree()
  We don't need any special handling.

BUG=none
TEST=none; no visible behavior changes.

Review URL: https://codereview.chromium.org/42903002

git-svn-id: svn://svn.chromium.org/blink/trunk@160526 bbb929c8-8fbe-4397-9dbb-9b2b20218538