Add CHECKs to track down why we're making frame sinks when we shouldn't

RenderWidgets attached to RenderView but without a main frame have no
business making a frame sink. Normally this is the state where they are
"swapped out" and we take great care to not make a frame sink in that
case by keeping the compositor non-visible. Something isn't working out
though so let's see why!

R=piman@chromium.org

Change-Id: Idc4e9e8184b6752f50a98a1efb82e436b1fcf08a
Bug: 896836
Reviewed-on: https://chromium-review.googlesource.com/c/1334002Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607785}

content/renderer/render_widget.cc

@@ -804,7 +804,9 @@ void RenderWidget::OnWasHidden() {
 void RenderWidget::OnWasShown(base::TimeTicks show_request_timestamp,
                               bool was_evicted) {
   TRACE_EVENT0("renderer", "RenderWidget::OnWasShown");
-  DCHECK(!is_swapped_out_);
+  // TODO(crbug.com/896836): CHECK to try track down why we make a frame sink
+  // for a RenderWidget without a main frame.
+  CHECK(!is_swapped_out_);
 
   // During shutdown we can just ignore this message.
   if (!GetWebWidget())
@@ -979,6 +981,15 @@ void RenderWidget::RequestNewLayerTreeFrameSink(
     return;
   }
 
+  // TODO(crbug.com/896836): CHECK to try track down why we make a frame sink
+  // for a RenderWidget without a main frame. If there's no frame widget then
+  // there is no main frame. This only applies for widgets for frames.
+  CHECK(!is_swapped_out_);
+  if (owner_delegate_)
+    CHECK(GetFrameWidget());
+  if (for_oopif_)
+    CHECK(GetFrameWidget());
+
   // TODO(jonross): have this generated by the LayerTreeFrameSink itself, which
   // would then handle binding.
   mojom::RenderFrameMetadataObserverPtr ptr;