Move the follwing members from HTMLPlugInImageElement to HTMLPlugInElement.

 - m_shouldPreferPlugInsForImages and shouldPreferPlugInsForImages()
 - isImageType()
 - m_serviceType
 - m_url

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

BUG=309978

Review URL: https://codereview.chromium.org/34623002

git-svn-id: svn://svn.chromium.org/blink/trunk@160185 bbb929c8-8fbe-4397-9dbb-9b2b20218538