Have the form control element directly handle validation message updating.

Remove the small ValidationMessage class, merging its functionality into
its owning form control element.

R=tkent@chromium.org
BUG=

Review URL: https://codereview.chromium.org/365223004

git-svn-id: svn://svn.chromium.org/blink/trunk@177527 bbb929c8-8fbe-4397-9dbb-9b2b20218538