Speculative fix for OnSwapCompositorFrame crasher.

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index cdb2d1799beccd2d50f0f35f1589df85cae22850..044ba949480b6483c81af940ed3a1ca61d87a270 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -283,7 +283,7 @@ void RenderWidgetHostViewGuest::OnSwapCompositorFrame(
   if (output_surface_id != last_output_surface_id_ ||
       frame_size != current_surface_size_ ||
       scale_factor != current_surface_scale_factor_ ||
-      guest_->has_attached_since_surface_set()) {
+      (guest_ && guest_->has_attached_since_surface_set())) {

[kenrb, 2016/09/16 13:37:31]

Would it make more sense to test guest_ at the beginning and early return?

[wjmaclean, 2016/09/16 13:47:23]

I wondered about that, but the comment at the end about "we should finish
processing it" reminded me about the earlier bug, which was fixed here:
https://codereview.chromium.org/1772393002

You can see that we got into different trouble when we had the early-out at the
beginning.

     ClearCompositorSurfaceIfNecessary();
     last_output_surface_id_ = output_surface_id;
     current_surface_size_ = frame_size;
@@ -305,8 +305,14 @@ void RenderWidgetHostViewGuest::OnSwapCompositorFrame(
     // SurfaceLayer.
     cc::SurfaceManager* manager = GetSurfaceManager();
     manager->GetSurfaceForId(surface_id_)->AddDestructionDependency(sequence);
-    guest_->SetChildFrameSurface(surface_id_, frame_size, scale_factor,
-                                 sequence);
+    // TODO(wjmaclean): I'm not sure what it means to create a surface id
+    // without setting it on the child, though since we will in this case be
+    // guaranteed to call ClearCompositorSurfaceIfNecessary() below, I suspect
+    // skipping SetChildFrameSurface() here is irrelevant.
+    if (guest_) {
+      guest_->SetChildFrameSurface(surface_id_, frame_size, scale_factor,
+                                   sequence);
+    }
   }
 
   cc::SurfaceFactory::DrawCallback ack_callback = base::Bind(