Plugin Snapshotting: Auto-start dominant plugins
​https://bugs.webkit.org/show_bug.cgi?id=113111
<rdar://problem/13475726>

Reviewed by Dean Jackson.

Add a test for http://trac.webkit.org/changeset/146679.

* platform/mac-wk2/plugins/snapshotting/autoplay-dominant-expected.txt: Added.
* plugins/snapshotting/autoplay-dominant.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146685 268f45cc-cd09-0410-ab3c-d52691b4dbfc