Remove addTextFieldDecoratorClient() from WebView API.

This is a deprecated method and marked to be removed when Chromium is updated.
Since Chromium was changed to call the new method in crrev.com/213789
(https://codereview.chromium.org/19597004/) this can be removed now.

R=abarth@chromium.org,esprehn@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21466002

git-svn-id: svn://svn.chromium.org/blink/trunk@155314 bbb929c8-8fbe-4397-9dbb-9b2b20218538