Skip invalidation rect update if empty

At ObjectPaintInvalidatorWithContext::InvalidateSelection we update PartialInvalidationVisualRect
even if the union rect of old and new selection rect is empty.

This patch skips the update if the union rect is empty.


Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I11dd813381b4fd5be69d4357e816d9c64cdb17ba
Reviewed-on: https://chromium-review.googlesource.com/1183011Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585346}

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

@@ -322,9 +322,13 @@ PaintInvalidationReason ObjectPaintInvalidatorWithContext::InvalidateSelection(
   if (full_invalidation)
     return reason;
 
+  const LayoutRect invalidation_rect =
+      UnionRect(new_selection_rect, old_selection_rect);
+  if (invalidation_rect.IsEmpty())
+    return reason;
+
   object_.GetMutableForPainting().SetPartialInvalidationVisualRect(
-      UnionRect(object_.PartialInvalidationVisualRect(),
-                UnionRect(new_selection_rect, old_selection_rect)));
+      UnionRect(object_.PartialInvalidationVisualRect(), invalidation_rect));
   return PaintInvalidationReason::kSelection;
 }