Move some members of HTMLPlugInImageElement to HTMLPlugInElement, part 2.

 - serviceType()
 - url()
 - m_loadedurl and loadedUrl()
 - loadedMimeType()
 - m_needsWidgetUpdate, needsWidgetUpdate(), and setNeedsWidgetUpdate()
 - updateWidget()

and remove HTMLPlugInImageElement::setDisplayState(), which just called
HTMLPlugInElement::setDisplayState().

This is a part of a task to merge HTMLPlugInImageELement into HTMLPlugInElement.
Only HTMLPlugInImageElement inherits from HTMLPlugInElement.

BUG=301217

Review URL: https://codereview.chromium.org/34903007

git-svn-id: svn://svn.chromium.org/blink/trunk@160296 bbb929c8-8fbe-4397-9dbb-9b2b20218538