Revert "Remove generic TextFieldDecorator in favor of PasswordGeneratorButtonElement"

This reverts commit bc1fa80104ff07789e0495c06381d56f3407428a.

This patch caused mac builder failures at least on chromium try bots.
http://build.chromium.org/p/tryserver.chromium/builders/mac/builds/69030

TBR=esprehn

Review URL: https://codereview.chromium.org/18254005

git-svn-id: svn://svn.chromium.org/blink/trunk@154104 bbb929c8-8fbe-4397-9dbb-9b2b20218538