Remove onBlocked() function from WebIDBCallbacks.

Looks like this functions is marked as deprecated and thus should be removed,
since there is no Chromium users we can remove this safely.

BUG=None
TEST=None, no functional change.
R=dgrogan@chromium.org,abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23475050

git-svn-id: svn://svn.chromium.org/blink/trunk@157969 bbb929c8-8fbe-4397-9dbb-9b2b20218538