PluginDocument should use AttachLazily

Use AttachLazily in all the appendChild calls in PluginDocument.

This doesn't really result in a real behavior change since PluginDocumentParser
calls updateLayout() right after appending the nodes so it can grab back the
Widget to pipe the stream of data off the network directly into the plugin.

Review URL: https://chromiumcodereview.appspot.com/22999028

git-svn-id: svn://svn.chromium.org/blink/trunk@156411 bbb929c8-8fbe-4397-9dbb-9b2b20218538