Revert of Remove stale RFHM check now that unload runs in the background.

content/browser/frame_host/render_frame_host_manager.cc

Index: content/browser/frame_host/render_frame_host_manager.cc
diff --git a/content/browser/frame_host/render_frame_host_manager.cc b/content/browser/frame_host/render_frame_host_manager.cc
index 3e23951027e74c96b274187200818fec9c0c0158..8d557ace789b3b3c784bab09ebaa946467840a5b 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -280,14 +280,17 @@
   // in progress.  Sanity check this for http://crbug.com/276333.
   CHECK(pending_render_frame_host_);
 
-  // Unload handlers run in the background, so we should never get an
-  // unresponsiveness warning for them.
-  CHECK(!render_frame_host_->IsWaitingForUnloadACK());
-
-  // If the tab becomes unresponsive during beforeunload while doing a
+  // If the tab becomes unresponsive during {before}unload while doing a
   // cross-site navigation, proceed with the navigation.  (This assumes that
   // the pending RenderFrameHost is still responsive.)
-  if (render_frame_host_->is_waiting_for_beforeunload_ack()) {
+  if (render_frame_host_->IsWaitingForUnloadACK()) {
+    // The request has been started and paused while we're waiting for the
+    // unload handler to finish.  We'll pretend that it did.  The pending
+    // renderer will then be swapped in as part of the usual DidNavigate logic.
+    // (If the unload handler later finishes, this call will be ignored because
+    // the pending_nav_params_ state will already be cleaned up.)
+    current_frame_host()->OnSwappedOut();
+  } else if (render_frame_host_->is_waiting_for_beforeunload_ack()) {
     // Haven't gotten around to starting the request, because we're still
     // waiting for the beforeunload handler to finish.  We'll pretend that it
     // did finish, to let the navigation proceed.  Note that there's a danger