Plug-ins that are appropriately large w.r.t page size should autostart
https://bugs.webkit.org/show_bug.cgi?id=111242

Reviewed by Brady Eidson.

A "full-page" plug-in site should never snapshot. The trick is
how to determine what is full-page. This change implements the
following algorithm.

- The plug-in is in the main frame (not an iframe).
- The plug-in is sized with width and height 100%.
- The displayed area of the plug-in is more than 96% of the viewport area.

This is definitely not foolproof. For example, zombo.com has a slight
border around its plug-in. As the window size gets smaller, the body margin
takes up more than 5% of the width or height, and the plug-in doesn't pass
the tests above.

* html/HTMLPlugInImageElement.cpp:
(WebCore): New static constant: sizingFullPageThresholdPercentage
(WebCore::HTMLPlugInImageElement::subframeLoaderWillCreatePlugIn): Implements
    the rules described above.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144577 268f45cc-cd09-0410-ab3c-d52691b4dbfc