Code cleanup for showValidationMessage

Removing showValidationMessage, as code on the chromium side has landed.

R=tkent@chromium.org
BUG=91638

Review URL: https://codereview.chromium.org/557753002

git-svn-id: svn://svn.chromium.org/blink/trunk@181697 bbb929c8-8fbe-4397-9dbb-9b2b20218538