Simplify and speed up painting of clipping masks

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

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index d4f28037c6037c13b25f0c1a00f6e12e7b95be38..285f2ded9a2edf22504834df2f8426c37d5c28fd 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -9,6 +9,7 @@
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
 #include "core/paint/LayerClipRecorder.h"
+#include "core/paint/LayoutObjectDrawingRecorder.h"
 #include "core/paint/ObjectPaintProperties.h"
 #include "core/paint/PaintInfo.h"
 #include "core/paint/PaintLayer.h"
@@ -901,6 +902,15 @@ void PaintLayerPainter::paintFragmentWithPhase(
                          paintFlags, clippingRule);
   }
 
+  // If we are painting a mask for any reason and we have already processed the
+  // clips, there is no need to go through the remaining painting pipeline.
+  // We know that the mask just needs the area bounded by the clip rects to be
+  // filled with black.
+  if (clipRecorder && phase == PaintPhaseClippingMask) {
+    fillMaskingFragment(context, *clipRecorder);
+    return;
+  }
+
   LayoutRect newCullRect(clipRect.rect());
   Optional<ScrollRecorder> scrollRecorder;
   LayoutPoint paintOffset = -m_paintLayer.layoutBoxLocation();
@@ -1116,4 +1126,25 @@ void PaintLayerPainter::paintOverlayScrollbars(
   m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
+void PaintLayerPainter::fillMaskingFragment(
+    GraphicsContext& context,
+    const LayerClipRecorder& clipRecorder) {
+  DCHECK(m_paintLayer.compositingState() == PaintsIntoOwnBacking);
+  DCHECK(m_paintLayer.layoutObject()->isBox());
+
+  const LayoutBox* layoutBox = toLayoutBox(m_paintLayer.layoutObject());
+  if (LayoutObjectDrawingRecorder::useCachedDrawingIfPossible(
+          context, *layoutBox, PaintPhaseClippingMask))
+    return;
+
+  IntRect clipBoundingRect = clipRecorder.snappedClipRect();
+  for (auto& roundedRect : clipRecorder.roundedRects()) {
+    IntRect snappedRect = enclosingIntRect(roundedRect.rect());
+    clipBoundingRect.unite(snappedRect);
+  }
+  LayoutObjectDrawingRecorder drawingRecorder(
+      context, *layoutBox, PaintPhaseClippingMask, clipBoundingRect);
+  context.fillRect(clipBoundingRect, Color::black);
+}
+
 }  // namespace blink