Add WebKit API for doing a hit-test that mimics GestureTap

Added blink::WebView::hitTestResultForTap to do so.
blink::WebView::hitTestResultAt will be removed in a follow up patch.

See also https://codereview.chromium.org/475633002/ on how chromium uses it.

BUG=403865

Review URL: https://codereview.chromium.org/470833002

git-svn-id: svn://svn.chromium.org/blink/trunk@185063 bbb929c8-8fbe-4397-9dbb-9b2b20218538