Only store previous clip rects for PaintLayers that support subsequences.

third_party/WebKit/Source/core/paint/PaintInvalidator.cpp

Index: third_party/WebKit/Source/core/paint/PaintInvalidator.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintInvalidator.cpp b/third_party/WebKit/Source/core/paint/PaintInvalidator.cpp
index a093214a08042650d511df00983008f1384d1dde..73e46ef079b3e8890b8f4b33f367668f1d743ad3 100644
--- a/third_party/WebKit/Source/core/paint/PaintInvalidator.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintInvalidator.cpp
@@ -502,6 +502,22 @@ void PaintInvalidator::InvalidatePaintIfNeeded(
     context.forced_subtree_invalidation_flags |=
         PaintInvalidatorContext::kForcedSubtreeVisualRectUpdate;
   }
+
+  bool usesCompositedScrolling = false;

[chrishtr, 2017/04/13 02:05:59]

FYI this code re-adds the logic removed in
https://codereview.chromium.org/2702883002/
which was removed expressly because it was
made redundant by the code in PrePaintTreeWalk.
Now that that code has been optimized, this has to come back.

+  if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled() &&
+      object.IsBoxModelObject() &&
+      ToLayoutBoxModelObject(object).UsesCompositedScrolling())
+    usesCompositedScrolling = true;
+  if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled() &&
+      context.old_visual_rect != object.VisualRect() &&
+      !usesCompositedScrolling
+      // Note that isLayoutView() below becomes unnecessary after the launch of
+      // root layer scrolling.
+      && (object.HasOverflowClip() || object.IsLayoutView())) {
+    // Changes to clipping may affect any descendant.
+    context.forced_subtree_invalidation_flags |=
+        PaintInvalidatorContext::kForcedSubtreeVisualRectUpdate;
+  }
 }
 
 void PaintInvalidator::ProcessPendingDelayedPaintInvalidations() {