Similar to bug 1650728; the ImageBridgeChild will do a sync-dispatch to create an ImageClient

This is problematic when the calling code is also on a background taskqueue as the background threadpool is made of a single thread.

For now we could make the runnable be dispatched with the NS_DISPATCH_EVENT_MAY_BLOCK flag; however it's rather counter-intuitive as the task dispatched isn't blocking on anything; it's the callee that is waiting and blocking until that task is running.

It seems to be a better approach to remove the need for the ImageClient to be created on the ImageBridgeChild thread.