Band-aid fix for bug 487406

There is certainly a deeper issue here, but since I expect RenderFrameProxyHost::render_frame_host_ to be disappearing very soon, this small fix seems okay to suppress the crash.

BUG=487406

Review URL: https://codereview.chromium.org/1134273002

Cr-Commit-Position: refs/heads/master@{#329867}

content/browser/frame_host/render_frame_host_manager.cc

@@ -937,6 +937,7 @@ bool RenderFrameHostManager::ClearProxiesInSiteInstance(
     // in the proxy) and it was still pending swap out, move the RFH to the
     // pending deletion list first.
     if (node->IsMainFrame() &&
+        proxy->render_frame_host() &&
         proxy->render_frame_host()->rfh_state() ==
         RenderFrameHostImpl::STATE_PENDING_SWAP_OUT) {
       scoped_ptr<RenderFrameHostImpl> swapped_out_rfh =

content/browser/frame_host/render_frame_proxy_host.cc

@@ -99,6 +99,7 @@ RenderViewHostImpl* RenderFrameProxyHost::GetRenderViewHost() {
 
 void RenderFrameProxyHost::TakeFrameHostOwnership(
     scoped_ptr<RenderFrameHostImpl> render_frame_host) {
+  CHECK(render_frame_host_ == nullptr);
   render_frame_host_ = render_frame_host.Pass();
   render_frame_host_->set_render_frame_proxy_host(this);
 }