PlzNavigate: improve tracking of improper subframe swap-out.

Upon a FrameTreeNode destruction a speculative RenderFrameHost might
be swapped-out instead of being destroyed along with the node. This
behavior differs from the current navigation implementation and was
causing a leak of a RenderFrameProxyHost.

This change adds a CHECK to stop the leak the same way
RenderFrameHostManager::SwapOutOldFrame does.

BUG=439423

Review URL: https://codereview.chromium.org/1066823004

Cr-Commit-Position: refs/heads/master@{#327044}

content/browser/frame_host/render_frame_host_manager.cc

@@ -663,6 +663,7 @@ void RenderFrameHostManager::DiscardUnusedFrame(
     // Any currently suspended navigations are no longer needed.
     render_frame_host->CancelSuspendedNavigations();
 
+    CHECK(!GetRenderFrameProxyHost(site_instance));
     RenderFrameProxyHost* proxy =
         new RenderFrameProxyHost(site_instance, frame_tree_node_);
     proxy_hosts_[site_instance->GetId()] = proxy;