SetNeedsRepaint for LayoutView's layer for crrev.com/759821

This is just a safety measure for some theoretical corner cases
that the bug may occur when the LayoutView and the document element
are not in the same composited layer. However, I haven't succeeded
to create such a test case yet.

TBR to catch up with the next canary build, so that this CL can be
merged together with crrev.com/759821.

TBR=pdr@chromium.org

Bug: 1070952
Change-Id: I1f8448da81aea18fc08515ec3f7b0b7d03bca197
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153904Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759909}

third_party/blink/renderer/core/paint/paint_property_tree_builder.cc

@@ -3527,6 +3527,7 @@ PaintPropertyChangeType PaintPropertyTreeBuilder::UpdateForSelf() {
       // SetBackgroundNeedsFullPaintInvalidation() won't work here because we
       // have already walked the LayoutView in PrePaintTreeWalk.
       LayoutView* layout_view = object_.View();
+      layout_view->Layer()->SetNeedsRepaint();
       auto reason = PaintInvalidationReason::kBackground;
       static_cast<const DisplayItemClient*>(layout_view)->Invalidate(reason);
       if (auto* scrollable_area = layout_view->GetScrollableArea()) {