Add text field change handling for autofill preview in textarea element.

Extend textarea setSuggestedValue() function to handle the dispaly when
initiate autofill from textarea element.

R=isherman@chromium.org, tkent@chromium.org
BUG=332557

Review URL: https://codereview.chromium.org/133443011

git-svn-id: svn://svn.chromium.org/blink/trunk@168720 bbb929c8-8fbe-4397-9dbb-9b2b20218538