Add a test which confirms a regression about input element focusing.

This test confirms that changing an input's value on blur event does not cause
 re-focus.

BUG=393168

Review URL: https://codereview.chromium.org/403533002

git-svn-id: svn://svn.chromium.org/blink/trunk@178423 bbb929c8-8fbe-4397-9dbb-9b2b20218538