Comment 10 for bug 243520

Created attachment 326964
Stop double instantiation even earlier.

This is based on Shane's earlier fix, but moves it even earlier in the process. The problem is that we end up in some cases instantiating the plugin from JS (due to JS content policy implementations or actions by the page etc), and then once the plugin frame is reflown, we don't notice that its plugin has already been instantiated so we re-instantiate (asynchronously). This patch makes us detect that the new frame already has a plugin in it, and doesn't even post the event to asynchronously instantiate the (already instantiated) plugin.

Bug 441424 has a good testcase for this problem, using an extension that contains a JS content policy implementation.

Shane, if you can test this patch as well (and Tristan too, if you have time to test yet another patch!), I'd really appreciate it.