Remove WebValidationMessageClient.

Blink requested validation message requests to both of
WebValidationMessageClient and WebViewClient functions. Chromium
browser has switched to the latter and we should remove
WebValidationMessageClient.

Content Shell still calls WebView::setValidationMessageClient() in order to
use a mock implementation. We keep WebView::
setValidationMessageClient() as an empty function for now, and remove it
later.

BUG=293608

Review URL: https://codereview.chromium.org/117133003

git-svn-id: svn://svn.chromium.org/blink/trunk@164077 bbb929c8-8fbe-4397-9dbb-9b2b20218538