Bug 1558390: For OOP iframes, also send the embedder accessible when the BrowserBridgeChild is created. r=yzen,nika

Previously, BrowserBridgeChild::SendSetEmbedderAccessible was only called when an OuterDocAccessible was constructed.
However, it's also possible that the BrowserBridgeChild is created *after* the OuterDocAccessible.
Therefore, we must also do this when a BrowserBridgeChild is created if the OuterDocAccessible already exists.

Differential Revision: https://phabricator.services.mozilla.com/D34474

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extra : moz-landing-system : lando