Blink: Replace all "plug-in" with "plugin".

ManualTests/plug-in-mutates-NSView-hierarchy-during-resize.html

 <style>
     div { padding: 10px; margin: 8px; }
     embed { width: 50px; height: 50px; }
     iframe { width: 50px; height: 50px; }
 </style>
 <script>
     var callCount = 0;
 
     function hideFirst()
     {
         if (++callCount != 2)
             return;
 
         var target = document.getElementById("target");
         if (!target)
             return;
 
         target.parentNode.removeChild(target);
         document.body.offsetTop;
     }
 </script>
 <p>
-    Test that plug-ins cannot cause the NSView hierarchy to change beneath <tt>-[NSView _invalidateGStatesForTree]</tt>.
+    Test that plugins cannot cause the NSView hierarchy to change beneath <tt>-[NSView _invalidateGStatesForTree]</tt>.
 </p>
 <p>
     To test, resize the browser window. Reload and repeat a few times. Each time you resize, the orange rectangle and
     its contents should vanish, and the browser should not crash.
 </p>
 <div id="target" style="border: solid orange;">
     <embed type="application/x-webkit-test-netscape"></embed>
     <iframe src=""></iframe>
     <iframe src=""></iframe>
     <iframe src=""></iframe>
 </div>
 <div style="border: solid blue;">
     <embed type="application/x-webkit-test-netscape" onsetwindow="hideFirst()"></embed>
 </div>