Make ValidationMessageClientImpl workable with null WebValidationMessageClient.

ValidationMessageClientImpl::m_client is optional. Also, WebViewImpl uses
a ValidationMessageClientImpl with null WebValidationMessageClient unless
WebView::setValidationMessageClient is called.

BUG=293608

Review URL: https://chromiumcodereview.appspot.com/24257005

git-svn-id: svn://svn.chromium.org/blink/trunk@158053 bbb929c8-8fbe-4397-9dbb-9b2b20218538