cros: Password echo support for keyboard driven OOBE.

- Add support to reveal last typed char in obscured text field backed
  by NativeTextfieldViews;
- Enable that for keyboard drive OOBE;

BUG=238290

Review URL: https://chromiumcodereview.appspot.com/16051005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@204794 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed