Fix remaining crashes where main frame is likely null

- Add null check for MainFrame() similar to elsewhere in
  VisualViewport::UpdatePaintPropertyNodesIfNeeded()

Bug: 1134597
Change-Id: Iaaf6d91788051c650f8ed270787cf47782649053
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468737
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816814}

third_party/blink/renderer/core/frame/visual_viewport.cc

@@ -325,7 +325,7 @@ PaintPropertyChangeType VisualViewport::UpdatePaintPropertyNodesIfNeeded(
   parent_property_tree_state_ =
       PropertyTreeStateOrAlias(*transform_parent, *clip_parent, *effect_parent);
 
-  if (change == PaintPropertyChangeType::kNodeAddedOrRemoved)
+  if (change == PaintPropertyChangeType::kNodeAddedOrRemoved && MainFrame())
     MainFrame()->View()->SetVisualViewportNeedsRepaint();
 
   return change;