Remove generic TextFieldDecorator in favor of PasswordGeneratorButtonElement

The generic support for text field decorators is only used by password generator
icons and also prevents us from allowing author ShadowRoots on <input>'s. It's
also not clear that we ever want to really allow lots of little icons inside inputs,
currently we'd allow 8 of them, but that seems crazy to stuff into inputs.

This is the first step of tightening up the interface for password generator just like
SpeechInputClient and allows removing 300 lines of code.

R=tkent@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18414005

git-svn-id: svn://svn.chromium.org/blink/trunk@154171 bbb929c8-8fbe-4397-9dbb-9b2b20218538