web: Remove now-unused WebView::dragSourceMovedTo().

The last call to this in chromium was removed in crrev.com/261623

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/227413006

git-svn-id: svn://svn.chromium.org/blink/trunk@171010 bbb929c8-8fbe-4397-9dbb-9b2b20218538