Add isHTMLPlugInElement() helpers for consistency with other HTML elements

Add isHTMLPlugInElement() helpers for consistency with other HTML elements.
Those helpers rely on the isPluginElement() virtual function to check if
an Element is an HTMLPlugInElement.

R=tkent, adamk
BUG=346095

Review URL: https://codereview.chromium.org/202153005

git-svn-id: svn://svn.chromium.org/blink/trunk@169589 bbb929c8-8fbe-4397-9dbb-9b2b20218538