Use recomputed interest rect only if it changed enough

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
index aa46e43940742b229900fa201650938a29b61626..7ce69eb6d7f25f0affdd2e3390a9f58aa5afadf3 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -800,21 +800,23 @@ static void paintScrollbar(const Scrollbar* scrollbar, GraphicsContext& context,
     scrollbar->paint(&context, CullRect(transformedClip));
 }
 
-void PaintLayerCompositor::paintContents(const GraphicsLayer* graphicsLayer, GraphicsContext& context, GraphicsLayerPaintingPhase, const IntRect* clip) const
+IntRect PaintLayerCompositor::paintContents(const GraphicsLayer* graphicsLayer, GraphicsContext& context, GraphicsLayerPaintingPhase, const IntRect* interestRect) const
 {
-    IntRect defaultClip;
-    if (RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled() && !clip) {
-        defaultClip.setSize(m_layoutView.layoutSize(IncludeScrollbars));
-        clip = &defaultClip;
+    IntRect defaultInterestRect;
+    if (RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled() && !interestRect) {
+        defaultInterestRect.setSize(m_layoutView.layoutSize(IncludeScrollbars));
+        interestRect = &defaultInterestRect;
     }
-    ASSERT(clip);
+    ASSERT(interestRect);
 
     if (graphicsLayer == layerForHorizontalScrollbar())
-        paintScrollbar(m_layoutView.frameView()->horizontalScrollbar(), context, *clip);
+        paintScrollbar(m_layoutView.frameView()->horizontalScrollbar(), context, *interestRect);
     else if (graphicsLayer == layerForVerticalScrollbar())
-        paintScrollbar(m_layoutView.frameView()->verticalScrollbar(), context, *clip);
+        paintScrollbar(m_layoutView.frameView()->verticalScrollbar(), context, *interestRect);
     else if (graphicsLayer == layerForScrollCorner())
-        FramePainter(*m_layoutView.frameView()).paintScrollCorner(&context, *clip);
+        FramePainter(*m_layoutView.frameView()).paintScrollCorner(&context, *interestRect);
+
+    return *interestRect;
 }
 
 bool PaintLayerCompositor::supportsFixedRootBackgroundCompositing() const