Remove unused member function WebViewClient::didChangeFormState()

This patch removes unused member function WebViewClient::didChangeFormState() and related test case.

BUG=302013
TEST=n/a; no behavior changes, existing test covers this change.
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26677002

git-svn-id: svn://svn.chromium.org/blink/trunk@159280 bbb929c8-8fbe-4397-9dbb-9b2b20218538