Add an accessibility setting to expose password values.

Android has a feature where if you have headphones plugged in, the screen
reader speaks the password text as you type it. Without headphones, it says
"dot dot dot". To implement this we need to get the password out of Blink.

BUG=399043

Review URL: https://codereview.chromium.org/704363004

git-svn-id: svn://svn.chromium.org/blink/trunk@184994 bbb929c8-8fbe-4397-9dbb-9b2b20218538