Partially revert an isolation boundary optimization.

This patch reverts a part of crrev.com/367a97eb. This seems to have
caused DCHECK failures on one of the telemetry tests.

R=chrishtr@chromium.org, pdr@chromium.org, wangxianzhu@chromium.org

Bug: 891270
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib99768154470fe69796e3a47645ffb1c981661a9
Reviewed-on: https://chromium-review.googlesource.com/c/1259298Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596325}

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

@@ -266,6 +266,8 @@ void PrePaintTreeWalk::InvalidatePaintLayerOptimizationsIfNeeded(
   paint_layer.SetPreviousPaintPhaseDescendantOutlinesEmpty(false);
   paint_layer.SetPreviousPaintPhaseFloatEmpty(false);
   paint_layer.SetPreviousPaintPhaseDescendantBlockBackgroundsEmpty(false);
+  context.paint_invalidator_context.subtree_flags |=
+      PaintInvalidatorContext::kSubtreeVisualRectUpdate;
 }
 
 bool PrePaintTreeWalk::NeedsTreeBuilderContextUpdate(