Move members of HTMLPlugInImageElement to HTMLPlugInElement, part 5.

This CL moves the following member functions:
 - requestObject()
 - shouldUsePlugin()
 - loadPlugin()
 - pluginIsLoadable()

and remove m_createdDuringUserGesture, which is never referred.

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

BUG=309978
R=hayato@chromium.org

Review URL: https://codereview.chromium.org/38973003

git-svn-id: svn://svn.chromium.org/blink/trunk@160410 bbb929c8-8fbe-4397-9dbb-9b2b20218538