Make a@ping work from context menus - blink side

This patch adds WebFrame::sendPings(), which takes a WebNode, checks whether it wraps an HTMLAnchorElement, and calls sendPings() on it if it does.

The chromium side of this change is https://codereview.chromium.org/384243004/

BUG=257281

Review URL: https://codereview.chromium.org/389693002

git-svn-id: svn://svn.chromium.org/blink/trunk@178101 bbb929c8-8fbe-4397-9dbb-9b2b20218538