create_web_extensions_webview() registers webextension:// URI scheme multiple times

create_web_extensions_webview() in ephy-web-extension-manager.c registers the webextension:// URI scheme multiple times on the same WebKitWebContext (in the !custom_web_context case). This is undefined behavior.

I landed this change recently to ensure we get a critical when this happens, since Evolution was making the same mistake. Previously, the new registration would clobber the original one. The new behavior is that the new registration is ignored. Passing a specific EphyWebExtension in the user data doesn't make sense either, since the handler is global to the web context and not specific to any particular extension. This needs to be cleaned up.