Move WebValidationMessageClient functions to WebViewClient.

We need to move an implmentation of WebValidationMessageClient from
chrome/ to content/, and don't need to have a dedicated interface.
ValidationMessageClientImpl calls both of WebValidationMessageClient
fucntions and WebViewClient functions until we complete the switch.

BUG=293608
TEST=none; no behavior changes.
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/23677005

git-svn-id: svn://svn.chromium.org/blink/trunk@158010 bbb929c8-8fbe-4397-9dbb-9b2b20218538