Fix crash related to showing a newly created window.

Previously the message would just get dropped if the opener frame was
discarded as it would be on a shutdown channel.

Ultimately this code should go away if we didn't have the
pending_contents_ list. I've created a separate issue for that because
this change likely needs to be merged into M88.

BUG=1149833, 1150976

Change-Id: If46b130a0c79a2e09ba7680ca6e3dfea881f39ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550250Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829841}

content/browser/renderer_host/render_frame_host_impl.cc

@@ -3646,6 +3646,15 @@ void RenderFrameHostImpl::ShowCreatedWindow(
   // the handle to this class's associated RenderWidgetHostView.
   RenderFrameHostImpl* opener_frame_host =
       FromFrameToken(GetProcess()->GetID(), opener_frame_token);
+
+  // If |opener_frame_host| has been destroyed just return.
+  // TODO(crbug.com/1150976): Get rid of having to look up the opener frame
+  // to find the newly created web contents, because it is actually just
+  // |delegate_|.
+  if (!opener_frame_host) {
+    std::move(callback).Run();
+    return;
+  }
   opener_frame_host->delegate()->ShowCreatedWindow(
       opener_frame_host, GetRenderWidgetHost()->GetRoutingID(), disposition,
       initial_rect, user_gesture);