Use isHTMLPlugInElement to replace checks 

All the checked Elements (<object>, <embed>, <applet>) derive from
HTMLPlugInElement and there is no other class deriving from it, so
use isHTMLPlugInElement here instead.

Review URL: https://codereview.chromium.org/413783002

git-svn-id: svn://svn.chromium.org/blink/trunk@178805 bbb929c8-8fbe-4397-9dbb-9b2b20218538