Don't clear or allocate paint properties if they won't change.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintPropertyTreeBuilder.h"
 
 #include <memory>
 #include "core/dom/DOMNodeIds.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 1174 matching lines @@
   bool needs_paint_properties =
       NeedsPaintOffsetTranslation(object) || NeedsTransform(object) ||
       NeedsEffect(object) || NeedsTransformForNonRootSVG(object) ||
       NeedsFilter(object) || NeedsCssClip(object) ||
       NeedsScrollbarPaintOffset(object) || NeedsOverflowScroll(object) ||
       NeedsPerspective(object) || NeedsSVGLocalToBorderBoxTransform(object) ||
       NeedsScrollTranslation(object) || NeedsCssClipFixedPosition(object);
 
   bool had_paint_properties = object.PaintProperties();
 
-  if (needs_paint_properties) {
+  if (needs_paint_properties && !had_paint_properties) {
     ObjectPaintProperties& paint_properties =
         object.GetMutableForPainting().EnsurePaintProperties();
-    if (!had_paint_properties &&
-        RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
       paint_properties.SetCompositorElementId(
           CreateDomNodeBasedCompositorElementId(object));

[pdr., 2017/05/15 19:52:19]

I think avoiding ClearPaintProperties is a nice cleanup but unlikely to affect
perf. OTOH, not calling CreateDomNodeBasedCompositorElementId as much might be a
perf win (although, spv2 only).

     }
-  } else {
+  } else if (!needs_paint_properties && had_paint_properties) {
     object.GetMutableForPainting().ClearPaintProperties();
-    if (had_paint_properties)
-      full_context.force_subtree_update = true;
+    full_context.force_subtree_update = true;
   }
 }
 
 void PaintPropertyTreeBuilder::UpdatePropertiesForSelf(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& full_context) {
   PaintPropertyTreeBuilderFragmentContext& context = full_context.fragments[0];
   if (object.IsSVGHiddenContainer()) {
     // SVG resources are painted within one or more other locations in the
     // SVG during paint, and hence have their own independent paint property
@@ skipping 56 matching lines @@
                            context.force_subtree_update);
 
     context.force_subtree_update |= object.SubtreeNeedsPaintPropertyUpdate();
   }
 
   if (object.CanContainAbsolutePositionObjects())
     context.container_for_absolute_position = &object;
 }
 
 }  // namespace blink