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

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

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
index 42d0cb93ce21beee0ffb1afea870788548b22afd..52c0126d2e5eb8ff17f389565e35e7d42199dd6f 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -1192,18 +1192,16 @@ void PaintPropertyTreeBuilder::UpdatePaintProperties(
 
   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;
   }
 }