Thanks to this:

  // FIXME: This is a temporary hack to ensure that we return a LocalFrame, even
  // when the mainFrame is remote.  FocusController needs to be refactored to
  // deal with RemoteFrames cross-process focus transfers.
  for (Frame* frame = &page_->MainFrame()->Tree().Top(); frame;
       frame = frame->Tree().TraverseNext()) {
    if (frame->IsLocalFrame() && ToLocalFrame(frame)->IsLocalRoot())
      return frame;
  }

this function essentially picks a random frame. We should see if we can reduce usage of this to zero.