Move members from HTMLPlugInImageElement to HTMLPlugInElement, part 3.

 - didMoveToNewDocument()
 - renderEmbeddedObject()
 - m_imageLoader
 - attach()
 - updateWidgetCallback
 - updateWidgetIfNecessary()

Also, this CL merges HTMLPlugInImageElement::detach() into
HTMLPlugInElement::detach().

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

BUG=309978

Review URL: https://codereview.chromium.org/36403003

git-svn-id: svn://svn.chromium.org/blink/trunk@160313 bbb929c8-8fbe-4397-9dbb-9b2b20218538