Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/skia/OpaqueRegionSkia.h

Index: Source/platform/graphics/skia/OpaqueRegionSkia.h
diff --git a/Source/platform/graphics/skia/OpaqueRegionSkia.h b/Source/platform/graphics/skia/OpaqueRegionSkia.h
index 14cb5ad67b91e11b7e9d7a5b33f5489e857ba287..b68255baf75f0970084bb036223f6289e45310ea 100644
--- a/Source/platform/graphics/skia/OpaqueRegionSkia.h
+++ b/Source/platform/graphics/skia/OpaqueRegionSkia.h
@@ -57,6 +57,9 @@ public:
 
     void setImageMask(const SkRect& imageOpaqueRect);
 
+    // Set this to true to track regions that occlude the destination instead of only regions that produce opaque pixels.
+    void setTreatOverwriteAsOpaque(bool value) { m_treatOverwriteAsOpaque = value; }
+
     enum DrawType {
         FillOnly,
         FillOrStroke
@@ -96,6 +99,7 @@ private:
     SkRect& currentTrackingOpaqueRect();
 
     SkRect m_opaqueRect;
+    bool m_treatOverwriteAsOpaque;
 
     Vector<CanvasLayerState, 3> m_canvasLayerStack;
 };